bug#25247: 26.0.50; Concurrency crashes with XLib

[Eli Zaretskii]

> Date: Fri, 30 Dec 2016 10:37:12 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: tino.calancha@gmail.com, 25247@debbugs.gnu.org
> 
> I will try to push a change that moves global lock acquiring into
> xgselect.c later today.

Done.  Tino, could you please try the latest master?

Elias, I'd be grateful if you could repeat your thread-related tests
from 2 weeks ago with the latest master (both in "emacs -nw" and in a
GUI session), and see if any of those problems are back.

Thanks.