Add check for libc++ headers found

Author: Meinersbur

llvm/cmake/modules/HandleLLVMStdlib.cmake

@@ -2,6 +2,7 @@
 # if the user has requested it.
 
 include(DetermineGCCCompatible)
+include(CheckIncludeFiles)
 
 if(NOT DEFINED LLVM_STDLIB_HANDLED)
   set(LLVM_STDLIB_HANDLED ON)
@@ -19,7 +20,11 @@ if(NOT DEFINED LLVM_STDLIB_HANDLED)
     if(LLVM_COMPILER_IS_GCC_COMPATIBLE)
       check_cxx_compiler_flag("-stdlib=libc++" CXX_COMPILER_SUPPORTS_STDLIB)
       check_linker_flag(CXX "-stdlib=libc++" CXX_LINKER_SUPPORTS_STDLIB)
-      if(CXX_COMPILER_SUPPORTS_STDLIB AND CXX_LINKER_SUPPORTS_STDLIB)
+      cmake_push_check_state()
+      set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -stdlib=libc++")
+      check_include_files("chrono" CXX_COMPILER_SUPPORTS_STDLIB_CHRONO LANGUAGE CXX)
+      cmake_pop_check_state()
+      if(CXX_COMPILER_SUPPORTS_STDLIB AND CXX_LINKER_SUPPORTS_STDLIB AND CXX_COMPILER_SUPPORTS_STDLIB_CHRONO)
         append("-stdlib=libc++"
           CMAKE_CXX_FLAGS CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS
           CMAKE_MODULE_LINKER_FLAGS)

llvm/cmake/modules/LLVMConfig.cmake.in

@@ -55,6 +55,8 @@ endif()
 
 set(LLVM_ENABLE_RTTI @LLVM_ENABLE_RTTI@)
 
+set(LLVM_ENABLE_LIBCXX @LLVM_ENABLE_LIBCXX@)
+
 set(LLVM_ENABLE_LIBEDIT @HAVE_LIBEDIT@)
 if(LLVM_ENABLE_LIBEDIT)
   find_package(LibEdit)