[cmake] Always respect existing CMAKE_REQUIRED_FLAGS when adding additional ones.

* Previously part of https://reviews.llvm.org/D41622.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@323171 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/Modules/CheckLibcxxAtomic.cmake

@@ -9,7 +9,7 @@ INCLUDE(CheckCXXSourceCompiles)
 
 function(check_cxx_atomics varname)
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS "-nodefaultlibs -std=c++11 -nostdinc++ -isystem ${LIBCXX_SOURCE_DIR}/include")
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs -std=c++11 -nostdinc++ -isystem ${LIBCXX_SOURCE_DIR}/include")
   if (${LIBCXX_GCC_TOOLCHAIN})
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
   endif()