bug#34618: 26.1.92; timerfd interface causes slowdown on Cygwin

[Eli Zaretskii]

> From: Ken Brown <kbrown@cornell.edu>
> CC: "34618@debbugs.gnu.org" <34618@debbugs.gnu.org>
> Date: Sat, 9 Mar 2019 19:27:34 +0000
> 
> The Cygwin problems with the timerfd functions have been fixed as of 
> cygwin-3.0.2, so I've re-enabled the use of these functions on the master 
> branch.  To play it safe, we should probably leave them disabled on the 
> emacs-26 
> branch.

Isn't there a way to get at the version of Cygwin at run time?  I
think the best solution is to allow timerfd functions only for
versions whose support is not buggy, and that's not a compile-time
decision, right?