_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
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=""