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

[Eli Zaretskii]

> From: Mattias Engdegård <mattias.engdegard@gmail.com>
> Date: Sun, 25 Dec 2022 15:32:06 +0100
> 
> The implementation of `json-available-p`,
> 
>   (and (fboundp 'json-serialize)
>        (condition-case nil
>            (json-serialize t)
>          (:success t)
>          (json-unavailable nil))))
> 
> probably isn't quite right on Windows: `json-serialize` is pure so it will be 
> called at compile time and the result, "true", used in the code (actually not 
> even that since the result is never used). Thus, if libjansson could not be 
> loaded during actual Emacs use (as opposed to when Emacs was built), this 
> would never be detected and json-available-p would still return t.

Yes, you are right.  But please come up with a smaller changeset which
only changes what strictly needs to be changed.  Or if you want, I can
do this myself.

AFAIU, we just need a C implementation of json-available-p, since
doing this in Lisp doesn't work.  So that's the change I expect,
nothing more, nothing less.  Like we do with other optional libraries
and features, there's more than enough examples in the codebase.

Thanks.