bug#40003: 27.0.70; Windows build is without jansson support

[Eli Zaretskii]

> From: Phillip Lord <phillip.lord@russet.org.uk>
> Cc: 40003@debbugs.gnu.org,  npostavs@gmail.com,  yyoncho@gmail.com
> Date: Tue, 30 Jun 2020 12:13:05 +0100
> 
> I have pushed a snapshot release of emacs-28 to alpha which contains
> this support as far as I can tell.

Thanks.

> "As far as I can tell" means
> 
> (fboundp 'json-serialize)
> 
> so any slightly more rigourous testing of this build against a real
> world use case would be appreciated.

There's a small test suite in test/src/json-tests.el.