Starting with cygwin-3.0.0, Cygwin has the timerfd functions.  But using
them causes Emacs to be very slow to respond to user input.  If I press
a key, there is a 1-2 second delay before Emacs responds.  I'm in the
process of trying to debug this
(http://www.cygwin.org/ml/cygwin/2019-02/msg00364.html).  In the
meantime, the attached patch disables the use of these functions.

OK for the emacs-26 branch (after filling in the bug number)?

Ken