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

[Eli Zaretskii]

> From: Nikolay Kudryavtsev <nikolay.kudryavtsev@gmail.com>
> Cc: rpluim@gmail.com, p.stephani2@gmail.com, 48228@debbugs.gnu.org
> Date: Thu, 6 May 2021 20:36:10 +0300
> 
> Eli is probably correct in that this would never happen in practice for 
> GMP, but what I'm saying is that it's a good idea to have a single 
> unified convention for testing every single dynamic library feature

GMP is different in this aspect from other libraries: Emacs built with
GMP will refuse to start if the GMP DLL is not available.