configure: enable glx-tls by default

configure: enable glx-tls by default

In the (not too) distant future we'd want to remove this option and
effectively drop the other codepath(s) we have in our dispatch.

Linux distributions have been using --enable-glx-tls for a number of
years. Some/most BSD platforms still don't support this, yet this should
serve as an encouragement to move things forwards.

Note: we had many bug reports were opened due to the wrong default
option. See the list below for details.

v2:
 - Correct default option in help string (Andreas)
 - Add bugzilla references.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70623
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=72902
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73778
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89043

Cc: Jean-Sébastien Pédron <dumbbell@FreeBSD.org>
Cc: Jonathan Gray <jsg@jsg.id.au>
Cc: mesa-maintainers@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Andreas Boll <andreas.boll.dev@gmail.com>