bug#49660: 28.0.50; json-available-p should exist

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 49660@debbugs.gnu.org
> Date: Tue, 20 Jul 2021 16:47:13 +0200
> 
> Lars Ingebrigtsen <larsi@gnus.org> writes:
> 
> > Yeah, I guess there's really no other cleanish way to do this? 
> 
> Oh, I thought of a simple hack: We could define json-available-p in
> json.c, but we could also have (say) loadup.el check whether it's
> defined, and if not, it can define a nil-returning stub?  That'll mean a
> whole lot less #ifdef-ing in json.c.

If you are ready to define json-available-p somewhere other than
json.c, then you could simply do that in subr.el or somesuch.  It
would be cleaner than defining stubs in loadup.el.