Do not inherit CMAKE_REQUIRED_FLAGS

Author: Meinersbur

llvm/cmake/modules/HandleLLVMStdlib.cmake

@@ -20,10 +20,16 @@ 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)
-      cmake_push_check_state()
+
+      # Check whether C++ include files are available
+      # runtimes/CMakeLists.txt adds -nostdlib++ and -nostdinc++ to
+      # CMAKE_REQUIRED_FLAGS, which are incompatible with -stdlib=libc++; use
+      # a fresh CMAKE_REQUIRED_FLAGS environment.
+      cmake_push_check_state(RESET)
       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