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/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()