Index: lib/builtins/CMakeLists.txt
===================================================================
--- lib/builtins/CMakeLists.txt
+++ lib/builtins/CMakeLists.txt
@@ -251,6 +251,7 @@
 if (NOT WIN32)
   foreach(arch x86_64 i386 arm)
     if(CAN_TARGET_${arch})
+      set_source_files_properties(${${arch}_SOURCES} PROPERTIES LANGUAGE C)
       add_compiler_rt_runtime(clang_rt.builtins-${arch} ${arch} STATIC
         SOURCES ${${arch}_SOURCES}
         CFLAGS "-std=c99")