diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -208,6 +208,14 @@
     )
 endif()
 
+# In some build configuraitons (like boostrapping clang), we need to be able to
+# install the libcxx headers before CMake configuraiton for libcxx runs. Making
+# the name of this target configurable allows LLVM/runtimes/CMakeLists.txt to
+# add this subdirectory to the LLVM build to put libcxx's headers in place
+# before libcxx's build configuration is run.
+if (NOT CXX_HEADER_TARGET)
+  set(CXX_HEADER_TARGET cxx-headers)
+endif()
 if(NOT LIBCXX_USING_INSTALLED_LLVM AND LIBCXX_HEADER_DIR)
   set(output_dir ${LIBCXX_HEADER_DIR}/include/c++/v1)
 
@@ -233,18 +241,18 @@
     list(APPEND out_files ${dst})
   endif()
 
-  add_custom_target(cxx-headers ALL DEPENDS ${out_files} ${LIBCXX_CXX_ABI_HEADER_TARGET})
+  add_custom_target(${CXX_HEADER_TARGET} ALL DEPENDS ${out_files} ${LIBCXX_CXX_ABI_HEADER_TARGET})
 else()
-  add_custom_target(cxx-headers)
+  add_custom_target(${CXX_HEADER_TARGET})
 endif()
-set_target_properties(cxx-headers PROPERTIES FOLDER "Misc")
+set_target_properties(${CXX_HEADER_TARGET} PROPERTIES FOLDER "Misc")
 
 if (LIBCXX_INSTALL_HEADERS)
   foreach(file ${files})
     get_filename_component(dir ${file} DIRECTORY)
     install(FILES ${file}
       DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}include/c++/v1/${dir}
-      COMPONENT cxx-headers
+      COMPONENT ${CXX_HEADER_TARGET}
       PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
     )
   endforeach()
@@ -255,19 +263,19 @@
       DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}include/c++/v1
       PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
       RENAME __config
-      COMPONENT cxx-headers)
+      COMPONENT ${CXX_HEADER_TARGET})
   endif()
 
   if (NOT CMAKE_CONFIGURATION_TYPES)
-    add_custom_target(install-cxx-headers
-                      DEPENDS cxx-headers ${generated_config_deps}
+    add_custom_target(install-${CXX_HEADER_TARGET}
+                      DEPENDS ${CXX_HEADER_TARGET} ${generated_config_deps}
                       COMMAND "${CMAKE_COMMAND}"
-                              -DCMAKE_INSTALL_COMPONENT=cxx-headers
+                              -DCMAKE_INSTALL_COMPONENT=${CXX_HEADER_TARGET}
                               -P "${CMAKE_BINARY_DIR}/cmake_install.cmake")
     # Stripping is a no-op for headers
-    add_custom_target(install-cxx-headers-stripped DEPENDS install-cxx-headers)
+    add_custom_target(install-${CXX_HEADER_TARGET}-stripped DEPENDS install-${CXX_HEADER_TARGET})
 
-    add_custom_target(install-libcxx-headers DEPENDS install-cxx-headers)
-    add_custom_target(install-libcxx-headers-stripped DEPENDS install-cxx-headers-stripped)
+    add_custom_target(install-libcxx-headers DEPENDS install-${CXX_HEADER_TARGET})
+    add_custom_target(install-libcxx-headers-stripped DEPENDS install-${CXX_HEADER_TARGET}-stripped)
   endif()
 endif()
diff --git a/llvm/runtimes/CMakeLists.txt b/llvm/runtimes/CMakeLists.txt
--- a/llvm/runtimes/CMakeLists.txt
+++ b/llvm/runtimes/CMakeLists.txt
@@ -4,8 +4,12 @@
 # should be built with the LLVM toolchain from the build directory. This file is
 # a first step to formalizing runtime build interfaces.
 
-# In the current state this file only works with compiler-rt, other runtimes
-# will work as the runtime build interface standardizes.
+# Setting CMake minimum required version should be at the very top of the file
+# if this is the entry point.
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+  cmake_minimum_required(VERSION 3.4.3)
+  project(Runtimes C CXX ASM)
+endif()
 
 # Find all subdirectories containing CMake projects
 file(GLOB entries *)
@@ -205,6 +209,11 @@
 
 else() # if this is included from LLVM's CMake
   include(LLVMExternalProjectUtils)
+  if (LLVM_EXTERNAL_LIBCXX_SOURCE_DIR)
+    set(LIBCXX_HEADER_DIR ${LLVM_INCLUDE_DIR}/c++/v1/)
+    set(CXX_HEADER_TARGET runtime-libcxx-headers)
+    add_subdirectory(${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR}/include ${CXX_HEADER_TARGET})
+  endif()
 
   if(NOT LLVM_BUILD_RUNTIMES)
     set(EXTRA_ARGS EXCLUDE_FROM_ALL)
@@ -354,7 +363,7 @@
 
     llvm_ExternalProject_Add(runtimes
                              ${CMAKE_CURRENT_SOURCE_DIR}
-                             DEPENDS ${ARG_DEPENDS}
+                             DEPENDS ${ARG_DEPENDS} runtime-libcxx-headers
                              # Builtins were built separately above
                              CMAKE_ARGS -DCOMPILER_RT_BUILD_BUILTINS=Off
                                         -DLLVM_INCLUDE_TESTS=${LLVM_INCLUDE_TESTS}
@@ -444,7 +453,7 @@
 
     llvm_ExternalProject_Add(runtimes-${name}
                              ${CMAKE_CURRENT_SOURCE_DIR}
-                             DEPENDS ${${name}_deps}
+                             DEPENDS ${${name}_deps} runtime-libcxx-headers
                              # Builtins were built separately above
                              CMAKE_ARGS -DCOMPILER_RT_BUILD_BUILTINS=Off
                                         -DLLVM_INCLUDE_TESTS=${LLVM_INCLUDE_TESTS}