bug#60311: json-available-p: make dynamically correct for Windows

[Eli Zaretskii]

> Cc: 60311@debbugs.gnu.org
> Date: Sun, 25 Dec 2022 17:40:47 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> 
> AFAIU, we just need a C implementation of json-available-p, since
> doing this in Lisp doesn't work.

This came out too ambiguous.  So here's a more accurate request:
please make your json_available_p use the same code as the first part
of the below:

  #ifdef WINDOWSNT
    if (!json_initialized)
      {
        Lisp_Object status;
        json_initialized = init_json_functions ();
        status = json_initialized ? Qt : Qnil;
        Vlibrary_cache = Fcons (Fcons (Qjson, status), Vlibrary_cache);
      }
    if (!json_initialized)
      Fsignal (Qjson_unavailable,
               list1 (build_unibyte_string ("jansson library not found")));
  #endif

and your ensure_json_available use the second part of the above.
There's no reason to reshuffle the code that works.

Also, please make ensure_json_available specific to WINDOWSNT, so that
we don't pay an extra function call on Posix platforms.

And finally, json--available-p should be defined on all platforms that
compile json.c, not just on WINDOWSNT; it should return t on Posix
platforms.

Thanks.