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

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Date: Wed, 20 Dec 2017 12:41:08 +0100
> Cc: 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
> 
> > I don't understand why. This function has been available since GnuTLS
> > was created, according to the docs. Do we know what GnuTLS version is used
> > in Travis CI?
> 
> It fails for me here with GnuTLS-3.4.10. According to
> https://lists.gnupg.org/pipermail/gnutls-help/2016-June/004129.html
> that function was added in 3.5.1. Looking through the git repo it was
> previously an internal function.

This means we cannot use this method without some fallback for systems
with older GnuTLS, as 3.5.X is still not old enough to assume it's
available widely enough.  Could we perhaps use a fixed list of
features for older GnuTLS versions?

Ted, could you please fix this ASAP?  It currently prevents the
release branch from building on many systems.