yacc.c: fix warnings about undefined macros

For instance with GCC 4.9 and --enable-gcc-warnings:

    25. input.at:1201: testing Torturing the Scanner ...
    ../../tests/input.at:1344: $CC $CFLAGS $CPPFLAGS  -c -o input.o input.c
    stderr:
    input.c:239:18: error: "__STDC_VERSION__" is not defined [-Werror=undef]
     # elif 199901 <= __STDC_VERSION__
                      ^
    input.c:256:18: error: "__STDC_VERSION__" is not defined [-Werror=undef]
     # elif 199901 <= __STDC_VERSION__
                      ^

* data/skeletons/yacc.c: Check that __STDC_VERSION__ is defined before
using it.

data/skeletons/yacc.c

@@ -419,7 +419,7 @@ typedef short yytype_int16;
 # elif defined ptrdiff_t && defined PTRDIFF_MAX
 #  define YYPTRDIFF_T ptrdiff_t
 #  define YYPTRDIFF_MAXIMUM PTRDIFF_MAX
-# elif 199901 <= __STDC_VERSION__
+# elif defined __STDC_VERSION__ && 199901 <= __STDC_VERSION__
 #  include <stddef.h> /* INFRINGES ON USER NAME SPACE */
 #  define YYPTRDIFF_T ptrdiff_t
 #  include <stdint.h> /* INFRINGES ON USER NAME SPACE */
@@ -436,7 +436,7 @@ typedef short yytype_int16;
 #  define YYSIZE_T __SIZE_TYPE__
 # elif defined size_t
 #  define YYSIZE_T size_t
-# elif 199901 <= __STDC_VERSION__
+# elif defined __STDC_VERSION__ && 199901 <= __STDC_VERSION__
 #  include <stddef.h> /* INFRINGES ON USER NAME SPACE */
 #  define YYSIZE_T size_t
 # else