Pass c99 to compiler in CMakeLists.txt

Fixes #3631

Signed-off-by: okhowang(王沛文) <okhowang@tencent.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0fccd3d..ec14024 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -170,6 +170,8 @@
 
 include(CheckCCompilerFlag)
 
+set(CMAKE_C_STANDARD 99)
+
 if(CMAKE_COMPILER_IS_GNU)
     # some warnings we want are not available with old GCC versions
     # note: starting with CMake 2.8 we could use CMAKE_C_COMPILER_VERSION