Cygwin: Don't define unused symbol "CYGWIN".  (__CYGWIN__ is used instead.)

PR: 51016
Submitted by: Carlo Bramini <carlo.bramix libero.it>
Reviewed by: trawick


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1529668 13f79535-47bb-0310-9956-ffa450edef68