bug#25061: consider adding %COMPAT to default gnutls priority string

[Eli Zaretskii]

> Date: Sun, 17 Dec 2017 05:39:22 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: wingo@igalia.com, 25061@debbugs.gnu.org, ludo@gnu.org,
>       p.stephani2@gmail.com, michael.albinus@gmx.de, larsi@gnus.org
> 
> > 1) undo the change (please don't just revert the commit)
> > 2) document the breakage
> > 3) start setting the default conditionally based on GnuTLS version (ugh)
> > 
> > Any other options?
> 
> If no other ideas come up, I don't see 3) as such a terrible choice.

Btw, you may wish to consider implementing the removal of %DUMBFW
inside gnutls.c, when the version of GnuTLS doesn't support that, this
could be cleaner on the Lisp level.