Is there a #define for C99?

#if __STDC_VERSION__ == 199901L
/* C99 */
#else
/* not C99 */
#endif

Change == to >= if you want to test for C99 and later.

There is not an specific #define value. Just check __STDC_VERSION__ and define it yourself! ;-)

#if __STDC_VERSION__ >= 199901L
/* C99 code */
#define C99
#else
/* Not C99 code */
#endif


#ifdef C99
/*My code in C99 format*/
#else
/*My code in C99 format*/
#endif

EDIT: A more general snippet, from here. I've just changed the defined names, just in case you'll use them a lot on the code:

#if defined(__STDC__)
# define C89
# if defined(__STDC_VERSION__)
#  define C90
#  if (__STDC_VERSION__ >= 199409L)
#   define C94
#  endif
#  if (__STDC_VERSION__ >= 199901L)
#   define C99
#  endif
#  if (__STDC_VERSION__ >= 201112L)
#   define C11
#  endif
# endif
#endif