Check for at least C99 at runtime.

c99_runtime_check.c

static int IsC99()
{
  return (2//* */2
          == 2);
}

main.c

#include <stdio.h>

int main()
{
  if (IsC99())
    printf("hello from C99 or beyond\n");
  else
    printf("hello from C89 or earlier\n");
  return 0;
}