bug#29147: 27.0.50; gnutls-available-p broken

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Pluim <rpluim@gmail.com>
> Date: Sun, 05 Nov 2017 11:54:57 +0100
> Cc: 29147@debbugs.gnu.org, Ted Zlatanov <tzz@lifelogs.com>
> 
> Juri Linkov <juri@linkov.net> writes:
> 
> > The Emacs GnuTLS manual says that gnutls-available-p returns t
> > if GnuTLS is available, but really it returns nil, and I don't see
> > how gnutls-available-p is supposed to return a non-nil value
> > when HAVE_GNUTLS is defined, but HAVE_GNUTLS3 is not.
> 
> Its docstring says:
> 
>     gnutls-available-p is a built-in function in `C source code'.
> 
>     (gnutls-available-p)
> 
>     Return list of capabilities if GnuTLS is available in this instance of Emacs.
> 
> I see that the info documentation says it returns t, so that needs
> fixing.

Not just the documentation, the code as well needs fixing.

> Your point about HAVE_GNUTLS3 is valid, that's true as of commit
> 583995c62dd424775dda33d5134ce04bee2ae685. Ted, was that intentional?

I cannot see how this could be intentional, because it makes it
impossible to distinguish between GnuTLS 2.x and no GnuTLS at all.  So
I fixed this on the release branch.

If Juri (or someone else) has GnuTLS 2.x, please test and close the
bug if it is indeed fixed.

Thanks.