Index: cmake/config-ix.cmake
===================================================================
--- cmake/config-ix.cmake
+++ cmake/config-ix.cmake
@@ -29,6 +29,9 @@
 check_cxx_compiler_flag(-fno-lto             COMPILER_RT_HAS_FNO_LTO_FLAG)
 check_cxx_compiler_flag(-msse3               COMPILER_RT_HAS_MSSE3_FLAG)
 
+# Language options
+check_c_compiler_flag(-std=c99 COMPILER_RT_HAS_STD_C99_FLAG)
+
 check_cxx_compiler_flag(/GR COMPILER_RT_HAS_GR_FLAG)
 check_cxx_compiler_flag(/GS COMPILER_RT_HAS_GS_FLAG)
 check_cxx_compiler_flag(/MT COMPILER_RT_HAS_MT_FLAG)
Index: lib/builtins/CMakeLists.txt
===================================================================
--- lib/builtins/CMakeLists.txt
+++ lib/builtins/CMakeLists.txt
@@ -361,11 +361,13 @@
         endif ()
       endforeach ()
 
+      append_string_if(COMPILER_RT_HAS_STD_C99_FLAG -std=c99 maybe_stdc99)
+
       add_compiler_rt_runtime(clang_rt.builtins
                               STATIC
                               ARCHS ${arch}
                               SOURCES ${${arch}_SOURCES}
-                              CFLAGS "-std=c99"
+                              CFLAGS ${maybe_stdc99}
                               PARENT_TARGET builtins)
     endif ()
   endforeach ()