diff --git a/compiler-rt/cmake/Modules/CustomLibcxx/CMakeLists.txt b/compiler-rt/cmake/Modules/CustomLibcxx/CMakeLists.txt
--- a/compiler-rt/cmake/Modules/CustomLibcxx/CMakeLists.txt
+++ b/compiler-rt/cmake/Modules/CustomLibcxx/CMakeLists.txt
@@ -2,7 +2,6 @@
 project(custom-libcxx C CXX)
 
 # Build static libcxxabi.
-set(LIBCXXABI_STANDALONE_BUILD 1)
 set(LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
 set(LIBCXXABI_ENABLE_EXCEPTIONS ON CACHE BOOL "")
 set(LIBCXXABI_HERMETIC_STATIC_LIBRARY ON CACHE STRING "")
@@ -11,7 +10,6 @@
 add_subdirectory(${COMPILER_RT_LIBCXXABI_PATH} ${CMAKE_CURRENT_BINARY_DIR}/cxxabi)
 
 # Build static libcxx without exceptions.
-set(LIBCXX_STANDALONE_BUILD 1)
 set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
 set(LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
 set(LIBCXX_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
diff --git a/compiler-rt/lib/fuzzer/CMakeLists.txt b/compiler-rt/lib/fuzzer/CMakeLists.txt
--- a/compiler-rt/lib/fuzzer/CMakeLists.txt
+++ b/compiler-rt/lib/fuzzer/CMakeLists.txt
@@ -161,11 +161,11 @@
                  -DCMAKE_POSITION_INDEPENDENT_CODE=ON
                  -DLIBCXXABI_ENABLE_EXCEPTIONS=OFF
                  -DLIBCXX_ABI_NAMESPACE=__Fuzzer)
-    target_compile_options(RTfuzzer.${arch} PRIVATE -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
+    target_compile_options(RTfuzzer.${arch} PRIVATE -cxx-isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
     add_dependencies(RTfuzzer.${arch} libcxx_fuzzer_${arch}-build)
-    target_compile_options(RTfuzzer_main.${arch} PRIVATE -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
+    target_compile_options(RTfuzzer_main.${arch} PRIVATE -cxx-isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
     add_dependencies(RTfuzzer_main.${arch} libcxx_fuzzer_${arch}-build)
-    target_compile_options(RTfuzzer_interceptors.${arch} PRIVATE -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
+    target_compile_options(RTfuzzer_interceptors.${arch} PRIVATE -cxx-isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
     add_dependencies(RTfuzzer_interceptors.${arch} libcxx_fuzzer_${arch}-build)
     partially_link_libcxx(fuzzer_no_main ${LIBCXX_${arch}_PREFIX} ${arch})
     partially_link_libcxx(fuzzer_interceptors ${LIBCXX_${arch}_PREFIX} ${arch})
diff --git a/compiler-rt/lib/fuzzer/tests/CMakeLists.txt b/compiler-rt/lib/fuzzer/tests/CMakeLists.txt
--- a/compiler-rt/lib/fuzzer/tests/CMakeLists.txt
+++ b/compiler-rt/lib/fuzzer/tests/CMakeLists.txt
@@ -64,7 +64,7 @@
      COMPILER_RT_LIBCXXABI_PATH)
     file(GLOB libfuzzer_headers ../*.h)
     set(LIBFUZZER_TEST_RUNTIME_DEPS libcxx_fuzzer_${arch}-build ${libfuzzer_headers})
-    set(LIBFUZZER_TEST_RUNTIME_CFLAGS -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
+    set(LIBFUZZER_TEST_RUNTIME_CFLAGS -cxx-isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
     set(LIBFUZZER_TEST_RUNTIME_LINK_FLAGS ${LIBCXX_${arch}_PREFIX}/lib/libc++.a)
   endif()
 
diff --git a/compiler-rt/lib/msan/tests/CMakeLists.txt b/compiler-rt/lib/msan/tests/CMakeLists.txt
--- a/compiler-rt/lib/msan/tests/CMakeLists.txt
+++ b/compiler-rt/lib/msan/tests/CMakeLists.txt
@@ -66,7 +66,7 @@
     KIND ${kind}
     COMPILE_DEPS ${MSAN_UNITTEST_HEADERS}
     DEPS gtest msan
-    CFLAGS -isystem ${CMAKE_CURRENT_BINARY_DIR}/../libcxx_msan_${arch}/include/c++/v1
+    CFLAGS -cxx-isystem ${CMAKE_CURRENT_BINARY_DIR}/../libcxx_msan_${arch}/include/c++/v1
            ${MSAN_UNITTEST_INSTRUMENTED_CFLAGS} ${cflags}
   )
 endmacro()
diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -403,9 +403,11 @@
 # Configure System
 #===============================================================================
 
+# TODO: Other projects rely on the location where we generate the libc++ headers
+#       internally. We should get rid of these dependencies.
 if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
   set(LIBCXX_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/${LLVM_DEFAULT_TARGET_TRIPLE}/c++)
-  set(LIBCXX_HEADER_DIR ${LLVM_BINARY_DIR})
+  set(LIBCXX_GENERATED_INCLUDE_DIR "${LLVM_BINARY_DIR}/include/c++/v1")
   set(LIBCXX_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX}/${LLVM_DEFAULT_TARGET_TRIPLE}/c++)
   if(LIBCXX_LIBDIR_SUBDIR)
     string(APPEND LIBCXX_LIBRARY_DIR /${LIBCXX_LIBDIR_SUBDIR})
@@ -413,11 +415,11 @@
   endif()
 elseif(LLVM_LIBRARY_OUTPUT_INTDIR)
   set(LIBCXX_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR})
-  set(LIBCXX_HEADER_DIR  ${LLVM_BINARY_DIR})
+  set(LIBCXX_GENERATED_INCLUDE_DIR "${LLVM_BINARY_DIR}/include/c++/v1")
   set(LIBCXX_INSTALL_LIBRARY_DIR lib${LIBCXX_LIBDIR_SUFFIX})
 else()
   set(LIBCXX_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LIBCXX_LIBDIR_SUFFIX})
-  set(LIBCXX_HEADER_DIR  ${CMAKE_BINARY_DIR})
+  set(LIBCXX_GENERATED_INCLUDE_DIR "${CMAKE_BINARY_DIR}/include/c++/v1")
   set(LIBCXX_INSTALL_LIBRARY_DIR lib${LIBCXX_LIBDIR_SUFFIX})
 endif()
 
diff --git a/libcxx/benchmarks/CMakeLists.txt b/libcxx/benchmarks/CMakeLists.txt
--- a/libcxx/benchmarks/CMakeLists.txt
+++ b/libcxx/benchmarks/CMakeLists.txt
@@ -10,7 +10,7 @@
 set(BENCHMARK_LIBCXX_COMPILE_FLAGS
     -Wno-unused-command-line-argument
     -nostdinc++
-    -isystem ${LIBCXX_SOURCE_DIR}/include
+    -cxx-isystem "${LIBCXX_GENERATED_INCLUDE_DIR}"
     -L${LIBCXX_LIBRARY_DIR}
     -Wl,-rpath,${LIBCXX_LIBRARY_DIR}
     ${SANITIZER_FLAGS}
@@ -20,7 +20,6 @@
           -L${LIBCXX_CXX_ABI_LIBRARY_PATH}
           -Wl,-rpath,${LIBCXX_CXX_ABI_LIBRARY_PATH})
 endif()
-list(APPEND BENCHMARK_LIBCXX_COMPILE_FLAGS -include "${LIBCXX_BINARY_DIR}/__config_site")
 split_list(BENCHMARK_LIBCXX_COMPILE_FLAGS)
 
 ExternalProject_Add(google-benchmark-libcxx
diff --git a/libcxx/cmake/Modules/HandleLibCXXABI.cmake b/libcxx/cmake/Modules/HandleLibCXXABI.cmake
--- a/libcxx/cmake/Modules/HandleLibCXXABI.cmake
+++ b/libcxx/cmake/Modules/HandleLibCXXABI.cmake
@@ -52,8 +52,10 @@
             COMMENT "Copying C++ ABI header ${fpath}...")
         list(APPEND abilib_headers "${dst}")
 
-        if (LIBCXX_HEADER_DIR)
-          set(dst "${LIBCXX_HEADER_DIR}/include/c++/v1/${dstdir}/${fpath}")
+        if (LIBCXX_GENERATED_INCLUDE_DIR) # always true
+          # TODO: libc++ shouldn't be responsible for copying the libc++abi
+          #       headers into the right location.
+          set(dst "${LIBCXX_GENERATED_INCLUDE_DIR}/include/c++/v1/${dstdir}/${fpath}")
           add_custom_command(OUTPUT ${dst}
               DEPENDS ${src}
               COMMAND ${CMAKE_COMMAND} -E copy_if_different ${src} ${dst}
diff --git a/libcxx/docs/TestingLibcxx.rst b/libcxx/docs/TestingLibcxx.rst
--- a/libcxx/docs/TestingLibcxx.rst
+++ b/libcxx/docs/TestingLibcxx.rst
@@ -37,6 +37,12 @@
   $ <build>/bin/llvm-lit -sv libcxx/test/std/depr/depr.c.headers/stdlib_h.pass.cpp # Run a single test
   $ <build>/bin/llvm-lit -sv libcxx/test/std/atomics libcxx/test/std/threads # Test std::thread and std::atomic
 
+In the default configuration, the tests are built against headers that form a
+fake installation root of libc++. This installation root has to be updated when
+changes are made to the headers, so you should re-run the `check-cxx-deps` target
+before running the tests manually with `lit` when you make any sort of change,
+including to the headers.
+
 Sometimes you'll want to change the way LIT is running the tests. Custom options
 can be specified using the `--param=<name>=<val>` flag. The most common option
 you'll want to change is the standard dialect (ie -std=c++XX). By default the
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -4,6 +4,7 @@
   __bits
   __bsd_locale_defaults.h
   __bsd_locale_fallbacks.h
+  __config
   __debug
   __errc
   __functional_03
@@ -189,62 +190,29 @@
   wctype.h
   )
 
-configure_file("__config_site.in"
-               "${LIBCXX_BINARY_DIR}/__config_site"
-               @ONLY)
+if (LIBCXX_GENERATED_INCLUDE_DIR) # Always true
+  configure_file("__config_site.in" "${LIBCXX_GENERATED_INCLUDE_DIR}/__config_site" @ONLY)
 
-# Generate a custom __config header. The new header is created
-# by prepending __config_site to the current __config header.
-add_custom_command(OUTPUT ${LIBCXX_BINARY_DIR}/__generated_config
-  COMMAND ${Python3_EXECUTABLE} ${LIBCXX_SOURCE_DIR}/utils/cat_files.py
-    ${LIBCXX_BINARY_DIR}/__config_site
-    ${LIBCXX_SOURCE_DIR}/include/__config
-    -o ${LIBCXX_BINARY_DIR}/__generated_config
-  DEPENDS ${LIBCXX_SOURCE_DIR}/include/__config
-          ${LIBCXX_BINARY_DIR}/__config_site
-)
-# Add a target that executes the generation commands.
-add_custom_target(cxx-generated-config ALL
-  DEPENDS ${LIBCXX_BINARY_DIR}/__generated_config)
-
-if(LIBCXX_HEADER_DIR)
-  set(output_dir ${LIBCXX_HEADER_DIR}/include/c++/v1)
-
-  set(out_files)
+  set(_all_includes "${LIBCXX_GENERATED_INCLUDE_DIR}/__config_site")
   foreach(f ${files})
-    set(src ${CMAKE_CURRENT_SOURCE_DIR}/${f})
-    set(dst ${output_dir}/${f})
+    set(src "${CMAKE_CURRENT_SOURCE_DIR}/${f}")
+    set(dst "${LIBCXX_GENERATED_INCLUDE_DIR}/${f}")
     add_custom_command(OUTPUT ${dst}
       DEPENDS ${src}
       COMMAND ${CMAKE_COMMAND} -E copy_if_different ${src} ${dst}
       COMMENT "Copying CXX header ${f}")
-    list(APPEND out_files ${dst})
+    list(APPEND _all_includes "${dst}")
   endforeach()
 
-  # Copy the generated header as __config into build directory.
-  set(src ${LIBCXX_BINARY_DIR}/__generated_config)
-  set(dst ${output_dir}/__config)
-  add_custom_command(OUTPUT ${dst}
-      DEPENDS ${src} cxx-generated-config
-      COMMAND ${CMAKE_COMMAND} -E copy_if_different ${src} ${dst}
-      COMMENT "Copying CXX __config")
-  list(APPEND out_files ${dst})
-  add_custom_target(generate-cxx-headers ALL DEPENDS ${out_files})
+  add_custom_target(generate-cxx-headers DEPENDS ${_all_includes})
 
   add_library(cxx-headers INTERFACE)
   add_dependencies(cxx-headers generate-cxx-headers ${LIBCXX_CXX_ABI_HEADER_TARGET})
   # TODO: Use target_include_directories once we figure out why that breaks the runtimes build
   if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC" OR "${CMAKE_CXX_SIMULATE_ID}" STREQUAL "MSVC")
-    target_compile_options(cxx-headers INTERFACE /I "${output_dir}")
-  else()
-    target_compile_options(cxx-headers INTERFACE -I "${output_dir}")
-  endif()
-
-  # Make sure the generated __config_site header is included when we build the library.
-  if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC" OR "${CMAKE_CXX_SIMULATE_ID}" STREQUAL "MSVC")
-    target_compile_options(cxx-headers INTERFACE /FI "${LIBCXX_BINARY_DIR}/__config_site")
+    target_compile_options(cxx-headers INTERFACE /I "${LIBCXX_GENERATED_INCLUDE_DIR}")
   else()
-    target_compile_options(cxx-headers INTERFACE -include "${LIBCXX_BINARY_DIR}/__config_site")
+    target_compile_options(cxx-headers INTERFACE -I "${LIBCXX_GENERATED_INCLUDE_DIR}")
   endif()
 else()
   add_library(cxx-headers INTERFACE)
@@ -260,16 +228,15 @@
     )
   endforeach()
 
-  # Install the generated header as __config.
-  install(FILES ${LIBCXX_BINARY_DIR}/__generated_config
+  # Install the generated __config_site.
+  install(FILES ${LIBCXX_GENERATED_INCLUDE_DIR}/__config_site
     DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}include/c++/v1
     PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
-    RENAME __config
     COMPONENT cxx-headers)
 
   if (NOT CMAKE_CONFIGURATION_TYPES)
     add_custom_target(install-cxx-headers
-                      DEPENDS cxx-headers cxx-generated-config
+                      DEPENDS cxx-headers
                       COMMAND "${CMAKE_COMMAND}"
                               -DCMAKE_INSTALL_COMPONENT=cxx-headers
                               -P "${CMAKE_BINARY_DIR}/cmake_install.cmake")
diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -10,6 +10,8 @@
 #ifndef _LIBCPP_CONFIG
 #define _LIBCPP_CONFIG
 
+#include <__config_site>
+
 #if defined(_MSC_VER) && !defined(__clang__)
 #  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #    define _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
diff --git a/libcxx/test/configs/legacy.cfg.in b/libcxx/test/configs/legacy.cfg.in
--- a/libcxx/test/configs/legacy.cfg.in
+++ b/libcxx/test/configs/legacy.cfg.in
@@ -3,6 +3,7 @@
 import os
 import site
 
+config.cxx_headers              = "@LIBCXX_GENERATED_INCLUDE_DIR@"
 config.cxx_under_test           = "@CMAKE_CXX_COMPILER@"
 config.project_obj_root         = "@CMAKE_BINARY_DIR@"
 config.libcxx_src_root          = "@LIBCXX_SOURCE_DIR@"
diff --git a/libcxx/utils/libcxx/test/config.py b/libcxx/utils/libcxx/test/config.py
--- a/libcxx/utils/libcxx/test/config.py
+++ b/libcxx/utils/libcxx/test/config.py
@@ -331,7 +331,6 @@
 
     def configure_compile_flags_header_includes(self):
         support_path = os.path.join(self.libcxx_src_root, 'test', 'support')
-        self.configure_config_site_header()
         if self.cxx_stdlib_under_test != 'libstdc++' and \
            not self.target_info.is_windows():
             self.cxx.compile_flags += [
@@ -348,16 +347,12 @@
                                          'set_windows_crt_report_mode.h')
             ]
         cxx_headers = self.get_lit_conf('cxx_headers')
-        if cxx_headers == '' or (cxx_headers is None
-                                 and self.cxx_stdlib_under_test != 'libc++'):
+        if cxx_headers is None and self.cxx_stdlib_under_test != 'libc++':
             self.lit_config.note('using the system cxx headers')
             return
         self.cxx.compile_flags += ['-nostdinc++']
-        if cxx_headers is None:
-            cxx_headers = os.path.join(self.libcxx_src_root, 'include')
         if not os.path.isdir(cxx_headers):
-            self.lit_config.fatal("cxx_headers='%s' is not a directory."
-                                  % cxx_headers)
+            self.lit_config.fatal("cxx_headers='{}' is not a directory.".format(cxx_headers))
         self.cxx.compile_flags += ['-I' + cxx_headers]
         if self.libcxx_obj_root is not None:
             cxxabi_headers = os.path.join(self.libcxx_obj_root, 'include',
@@ -365,16 +360,6 @@
             if os.path.isdir(cxxabi_headers):
                 self.cxx.compile_flags += ['-I' + cxxabi_headers]
 
-    def configure_config_site_header(self):
-        # Check for a possible __config_site in the build directory. We
-        # use this if it exists.
-        if self.libcxx_obj_root is None:
-            return
-        config_site_header = os.path.join(self.libcxx_obj_root, '__config_site')
-        if not os.path.isfile(config_site_header):
-            return
-        self.cxx.compile_flags += ['-include', config_site_header]
-
     def configure_link_flags(self):
         # Configure library path
         self.configure_link_flags_cxx_library_path()
diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -155,9 +155,23 @@
   message(FATAL_ERROR "libc++abi must be built as either a shared or static library.")
 endif()
 
-set(LIBCXXABI_LIBCXX_INCLUDES "${LIBCXXABI_LIBCXX_PATH}/include" CACHE PATH
-    "Specify path to libc++ includes.")
-message(STATUS "Libc++abi will be using libc++ includes from ${LIBCXXABI_LIBCXX_INCLUDES}")
+# TODO: This is a hack for the fact that Standalone builds can't use targets
+#       from the other runtimes (so the cxx-headers target doesn't exist).
+set(LIBCXXABI_LIBCXX_INCLUDES "" CACHE PATH
+    "Specify path to the installed libc++ includes. This is only relevant for Standalone builds, where the location of the headers can't be determined automatically.")
+if (LIBCXXABI_STANDALONE_BUILD)
+  if (NOT IS_DIRECTORY ${LIBCXXABI_LIBCXX_INCLUDES})
+    message(FATAL_ERROR
+      "LIBCXXABI_LIBCXX_INCLUDES=${LIBCXXABI_LIBCXX_INCLUDES} is not a valid directory. "
+      "Please provide the path to where the libc++ headers have been installed.")
+  endif()
+  add_library(cxx-headers INTERFACE)
+  if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC" OR "${CMAKE_CXX_SIMULATE_ID}" STREQUAL "MSVC")
+    target_compile_options(cxx-headers INTERFACE /I "${LIBCXXABI_LIBCXX_INCLUDES}")
+  else()
+    target_compile_options(cxx-headers INTERFACE -I "${LIBCXXABI_LIBCXX_INCLUDES}")
+  endif()
+endif()
 
 option(LIBCXXABI_HERMETIC_STATIC_LIBRARY
   "Do not export any symbols from the static library." OFF)
diff --git a/libcxxabi/src/CMakeLists.txt b/libcxxabi/src/CMakeLists.txt
--- a/libcxxabi/src/CMakeLists.txt
+++ b/libcxxabi/src/CMakeLists.txt
@@ -55,8 +55,6 @@
   endif()
 endif()
 
-include_directories("${LIBCXXABI_LIBCXX_INCLUDES}")
-
 # stdlib_stdexcept.cpp depends on libc++ internals.
 include_directories("${LIBCXXABI_LIBCXX_PATH}")
 
@@ -177,7 +175,7 @@
 # Build the shared library.
 if (LIBCXXABI_ENABLE_SHARED)
   add_library(cxxabi_shared SHARED ${LIBCXXABI_SOURCES} ${LIBCXXABI_HEADERS})
-  target_link_libraries(cxxabi_shared PRIVATE ${LIBCXXABI_SHARED_LIBRARIES} ${LIBCXXABI_LIBRARIES})
+  target_link_libraries(cxxabi_shared PRIVATE cxx-headers ${LIBCXXABI_SHARED_LIBRARIES} ${LIBCXXABI_LIBRARIES})
   if (TARGET pstl::ParallelSTL)
     target_link_libraries(cxxabi_shared PUBLIC pstl::ParallelSTL)
   endif()
@@ -244,7 +242,7 @@
 # Build the static library.
 if (LIBCXXABI_ENABLE_STATIC)
   add_library(cxxabi_static STATIC ${LIBCXXABI_SOURCES} ${LIBCXXABI_HEADERS})
-  target_link_libraries(cxxabi_static PRIVATE ${LIBCXXABI_STATIC_LIBRARIES} ${LIBCXXABI_LIBRARIES})
+  target_link_libraries(cxxabi_static PRIVATE cxx-headers ${LIBCXXABI_STATIC_LIBRARIES} ${LIBCXXABI_LIBRARIES})
   if (TARGET pstl::ParallelSTL)
     target_link_libraries(cxxabi_static PUBLIC pstl::ParallelSTL)
   endif()
diff --git a/libcxxabi/test/libcxxabi/test/config.py b/libcxxabi/test/libcxxabi/test/config.py
--- a/libcxxabi/test/libcxxabi/test/config.py
+++ b/libcxxabi/test/libcxxabi/test/config.py
@@ -56,10 +56,8 @@
         super(Configuration, self).configure_compile_flags()
 
     def configure_compile_flags_header_includes(self):
-        self.configure_config_site_header()
-        cxx_headers = self.get_lit_conf(
-            'cxx_headers',
-            os.path.join(self.libcxx_src_root, '/include'))
+        cxx_headers = self.get_lit_conf('cxx_headers', None) or \
+                    os.path.join(self.project_obj_root, 'include', 'c++', 'v1')
         if cxx_headers == '':
             self.lit_config.note('using the systems c++ headers')
         else:
diff --git a/libunwind/test/libunwind/test/config.py b/libunwind/test/libunwind/test/config.py
--- a/libunwind/test/libunwind/test/config.py
+++ b/libunwind/test/libunwind/test/config.py
@@ -52,8 +52,6 @@
         super(Configuration, self).configure_compile_flags()
 
     def configure_compile_flags_header_includes(self):
-        self.configure_config_site_header()
-
         libunwind_headers = self.get_lit_conf(
             'libunwind_headers',
             os.path.join(self.libunwind_src_root, 'include'))
diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -82,10 +82,6 @@
 # these flags.
 if (NOT MSVC)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++ -nostdlib++")
-
-  if (LLVM_EXTERNAL_LIBCXX_SOURCE_DIR AND "libcxx" IN_LIST LLVM_ENABLE_RUNTIMES)
-    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -isystem ${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR}/include")
-  endif()
 endif()
 
 # Avoid checking whether the compiler is working.
@@ -96,9 +92,6 @@
 include(HandleLLVMOptions)
 include(FindPythonInterp)
 
-# Remove the -nostdlib++ option we've added earlier.
-string(REPLACE "-nostdlib++" "" CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS}")
-
 # Use libtool instead of ar if you are both on an Apple host, and targeting Apple.
 if(CMAKE_HOST_APPLE AND APPLE)
   include(UseLibtool)
@@ -114,12 +107,10 @@
   string(REPLACE "-" "_" canon_name ${projName})
   string(TOUPPER ${canon_name} canon_name)
 
-  # The subdirectories need to treat this as standalone builds. D57992 tried
-  # to get rid of this, but the runtimes treat *_STANDALONE_BUILD=OFF as if
-  # llvm & clang are configured in the same CMake, and setup dependencies
-  # against their targets. OpenMP has fixed the issue so we don't set the
-  # variable.
-  if (NOT ${canon_name} STREQUAL "OPENMP")
+  # compiler-rt needs to treat this as standalone builds. D57992 tried
+  # to get rid of it for compiler-rt, but it treats *_STANDALONE_BUILD=OFF
+  # as if llvm & clang are configured in the same CMake, and setup dependencies
+  if ("${canon_name}" STREQUAL "COMPILER_RT")
     set(${canon_name}_STANDALONE_BUILD ON)
   endif()