[CMake] Passthrough CFLAGS when checking the compiler-rt path

This is needed when cross-compiling for a different target since
CFLAGS may contain additional flags like -resource-dir which
change the location in which compiler-rt builtins are found.

Differential Revision: https://reviews.llvm.org/D54371

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@346820 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/Modules/HandleCompilerRT.cmake

@@ -8,6 +8,9 @@ function(find_compiler_rt_library name dest)
   if (CMAKE_CXX_COMPILER_ID MATCHES Clang AND CMAKE_CXX_COMPILER_TARGET)
     list(APPEND CLANG_COMMAND "--target=${CMAKE_CXX_COMPILER_TARGET}")
   endif()
+  get_property(LIBCXX_CXX_FLAGS CACHE CMAKE_CXX_FLAGS PROPERTY VALUE)
+  string(REPLACE " " ";" LIBCXX_CXX_FLAGS "${LIBCXX_CXX_FLAGS}")
+  list(APPEND CLANG_COMMAND ${LIBCXX_CXX_FLAGS})
   execute_process(
       COMMAND ${CLANG_COMMAND}
       RESULT_VARIABLE HAD_ERROR