Fix PR26622 - Make CheckLibcxxAtomic.cmake use the libc++ headers.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@261383 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/Modules/CheckLibcxxAtomic.cmake

@@ -9,9 +9,9 @@ INCLUDE(CheckCXXSourceCompiles)
 
 function(check_cxx_atomics varname)
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS "-std=c++11")
+  set(CMAKE_REQUIRED_FLAGS "-std=c++11 -nostdinc++ -isystem ${LIBCXX_SOURCE_DIR}/include")
   if (${LIBCXX_GCC_TOOLCHAIN})
-    set(CMAKE_REQUIRED_FLAGS "-std=c++11 --gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
   endif()
   check_cxx_source_compiles("
 #include <cstdint>