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

[Eli Zaretskii]

> From: Nikolay Kudryavtsev <nikolay.kudryavtsev@gmail.com>
> Cc: dgutov@yandex.ru, rpluim@gmail.com, p.stephani2@gmail.com,
>  48228@debbugs.gnu.org
> Date: Thu, 6 May 2021 19:13:32 +0300
> 
> Oh, cool.
> 
> I think only GMP is left without a clear way to test for then.

The GMP test isn't necessary, because Emacs offers the same
functionality even without it, using a Gnulib module.

The only library I know of that doesn't have a test is libjansson.