Index: lib/builtins/CMakeLists.txt
===================================================================
--- lib/builtins/CMakeLists.txt
+++ lib/builtins/CMakeLists.txt
@@ -407,12 +407,11 @@
           list(REMOVE_ITEM ${arch}_SOURCES ${_cname})
         endif ()
       endforeach ()
-
       add_compiler_rt_runtime(clang_rt.builtins
                               STATIC
                               ARCHS ${arch}
                               SOURCES ${${arch}_SOURCES}
-                              CFLAGS ${maybe_stdc99}
+                              CFLAGS ${maybe_stdc99} ${CMAKE_C_FLAGS}
                               PARENT_TARGET builtins)
     endif ()
   endforeach ()