Index: lib/builtins/CMakeLists.txt
===================================================================
--- lib/builtins/CMakeLists.txt
+++ lib/builtins/CMakeLists.txt
@@ -361,11 +361,18 @@
         endif ()
       endforeach ()
 
+      # MSVC doesn't take this argument, silence the warning
+      if (MSVC)
+        set(_cflags "")
+      else ()
+        set(_cflags "-std=c99")
+      endif ()
+
       add_compiler_rt_runtime(clang_rt.builtins
                               STATIC
                               ARCHS ${arch}
                               SOURCES ${${arch}_SOURCES}
-                              CFLAGS "-std=c99"
+                              CFLAGS ${_cflags}
                               PARENT_TARGET builtins)
     endif ()
   endforeach ()