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

[Eli Zaretskii]

> From: Ted Zlatanov <tzz@lifelogs.com>
> Cc: Andy Moreton <andrewjmoreton@gmail.com>,  Noam Postavsky 
> <npostavs@users.sourceforge.net>,  wingo@igalia.com,  25061@debbugs.gnu.org,  
> ludo@gnu.org,  p.stephani2@gmail.com,  michael.albinus@gmx.de,  larsi@gnus.org
> Date: Wed, 20 Dec 2017 20:39:58 -0500
> 
> Noah's Travis CI build passed. I'll wait for Andy, Eli, and Kaushal to
> confirm before marking this as done.

The release branch builds OK now, thanks.  I've successfully visited
Savannah via HTTPS.  Should I test anything else to make sure GnuTLS
works as expected?