Use HAVE_STDINT_H instead of trying to guess based on __STDC_VERSION__.

Fixes compilation with pre-C99 headers when the compiler supports C99.

plugins/sudoers/gram.c

@@ -488,7 +488,7 @@ typedef enum yysymbol_kind_t yysymbol_kind_t;
 
 #ifndef __PTRDIFF_MAX__
 # include <limits.h> /* INFRINGES ON USER NAME SPACE */
-# if defined __STDC_VERSION__ && 199901 <= __STDC_VERSION__
+# if defined HAVE_STDINT_H
 #  include <stdint.h> /* INFRINGES ON USER NAME SPACE */
 #  define YY_STDINT_H
 # endif