Index: compiler-rt/lib/crt/CMakeLists.txt
===================================================================
--- compiler-rt/lib/crt/CMakeLists.txt
+++ compiler-rt/lib/crt/CMakeLists.txt
@@ -71,6 +71,7 @@
 check_cxx_section_exists(".init_array" COMPILER_RT_HAS_INITFINI_ARRAY
   SOURCE "__attribute__((constructor)) void f() {}\nint main() { return 0; }\n")
 
+set(CRT_CFLAGS -std=c99)
 append_list_if(COMPILER_RT_HAS_INITFINI_ARRAY -DCRT_HAS_INITFINI_ARRAY CRT_CFLAGS)
 append_list_if(COMPILER_RT_HAS_FPIC_FLAG -fPIC CRT_CFLAGS)
 append_list_if(COMPILER_RT_HAS_WNO_PEDANTIC -Wno-pedantic CRT_CFLAGS)