Index: compiler-rt/trunk/cmake/config-ix.cmake
===================================================================
--- compiler-rt/trunk/cmake/config-ix.cmake
+++ compiler-rt/trunk/cmake/config-ix.cmake
@@ -28,6 +28,7 @@
 check_cxx_compiler_flag(-ftls-model=initial-exec COMPILER_RT_HAS_FTLS_MODEL_INITIAL_EXEC)
 check_cxx_compiler_flag(-fno-lto             COMPILER_RT_HAS_FNO_LTO_FLAG)
 check_cxx_compiler_flag(-msse3               COMPILER_RT_HAS_MSSE3_FLAG)
+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)
Index: compiler-rt/trunk/lib/builtins/CMakeLists.txt
===================================================================
--- compiler-rt/trunk/lib/builtins/CMakeLists.txt
+++ compiler-rt/trunk/lib/builtins/CMakeLists.txt
@@ -359,6 +359,8 @@
   add_subdirectory(macho_embedded)
   darwin_add_builtin_libraries(${BUILTIN_SUPPORTED_OS})
 elseif (NOT WIN32 OR MINGW)
+  append_string_if(COMPILER_RT_HAS_STD_C99_FLAG -std=c99 maybe_stdc99)
+
   foreach (arch ${BUILTIN_SUPPORTED_ARCH})
     if (CAN_TARGET_${arch})
       # Filter out generic versions of routines that are re-implemented in
@@ -376,7 +378,7 @@
                               STATIC
                               ARCHS ${arch}
                               SOURCES ${${arch}_SOURCES}
-                              CFLAGS "-std=c99"
+                              CFLAGS ${maybe_stdc99}
                               PARENT_TARGET builtins)
     endif ()
   endforeach ()