build failure without system extension macros

[Grisha Levit]

Since 9c430f6b changed some files to no longer include config.h,
building with glibc and an -std= in CFLAGS fails because getopt,
used in support/bashversion.c and mksyntax.c, is guarded by
_POSIX_C_SOURCE >= 2 || _XOPEN_SOURCE

support/bashversion.c:69:17: warning: implicit declaration of function 'getopt' 
[-Wimplicit-function-declaration]
   69 |   while ((opt = getopt(argc, argv, "hrvmpslx")) != EOF)
      |                 ^~~~~~
support/bashversion.c:103:11: error: 'optind' undeclared (first use in this 
function)
  103 |   argc -= optind;
      |           ^~~~~~