Hello,

I was trying to give the Emacs 27 pre-test a spin, notably for the
native JSON support, and it took me a long time to figure out it
wasn't actually being configured for it as I was missing the Jansson
library headers.

This patch explicitly fails ./configure if `--with-json' is passed but
cannot be fulfilled. Which means users without the Jansson headers
won't be able to compile Emacs unless explicitly disabling it with
`--without-json'; I don't know if that is too extreme, maybe
AC_MSG_WARN would be more appropriate, but as long as it's notified in
some way to the user I'm happy.

It's also my very first patch so please excuse any mistakes in the
protocol/patch.

Cheers,
Stéphane.