Fix compiler warning for ppoll() on Cygwin

_GNU_SOURCE is required to get the prototype, so just define that
globally, as was already done in the linux template.

Discussion: https://www.postgresql.org/message-id/flat/6b467edc-4018-521f-ab18-171f098557ca%402ndquadrant.com

src/template/cygwin

@@ -2,6 +2,9 @@
 
 SRCH_LIB="/usr/local/lib"
 
+# This is required for ppoll(2), and perhaps other things
+CPPFLAGS="$CPPFLAGS -D_GNU_SOURCE"
+
 # Extra CFLAGS for code that will go into a shared library
 CFLAGS_SL=""