Pass c99 to compiler in CMakeLists.txt

Fixes #3631

Signed-off-by: okhowang(王沛文) <okhowang@tencent.com>

3rdparty/everest/library/Hacl_Curve25519_joined.c

@@ -18,6 +18,12 @@
  *
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
+#ifndef _BSD_SOURCE
+#define _BSD_SOURCE
+#endif
+#ifndef _DEFAULT_SOURCE
+#define _DEFAULT_SOURCE
+#endif
 
 #include "common.h"
 

CMakeLists.txt

@@ -170,6 +170,8 @@ string(REGEX MATCH "Clang" CMAKE_COMPILER_IS_CLANG "${CMAKE_C_COMPILER_ID}")
 
 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