bug#48228: json-serialize should signal error when dll is not found [MS Windows]

[Eli Zaretskii]

> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Tue, 4 May 2021 18:49:14 +0300
> Cc: 48228@debbugs.gnu.org
> 
> On 04.05.2021 18:46, Philipp Stephani wrote:
> > Maybe json-unavailable? json.c already defines several other error symbols.
> 
> It might make sense to define a common error that other features (like 
> xml.c) would use in similar situation.

An alternative to signaling an error would be to provide a
json-avail-p functon which applications could test.  That would be
similar to what we do with other libraries, like image libraries and
GnuTLS.