bug#47159: 28.0.50; gnutls not recognized in build process without pkg-config

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Thu, 18 Mar 2021 06:52:16 +0100
> Cc: 47159@debbugs.gnu.org
> 
> So I guess we should just add a check for pkg-config in the gnutls
> check?  Any opinions?

I think the check is there already, we just don't expose the results
to the user, and instead claim that whatever feature we needed
pkg-config for is not available.  IOW, what is missing here is the
message saying pkg-config wasn't found, and therefore a test for
FEATURE couldn't be done.