Ensure _GNU_SOURCE is now longer defined on powerpc64 (which was a
regression from the GCC 8 series).
The technical background is that a consolidation in upstream GCC made
non-GNU platforms include gnu-user.h and then undefined some macros
in rs6000/freebsd.h, but missed doing the same in rs6000/freebsd64.h.
The has now been included upstream and the current snapshot that the
lang/gcc9-devel port tracks; carrying files/patch-powerpc64-no-_GNU_SOURCE
in this port should become obsolete with the GCC 9.2 release.
(As this should be a very short-lived measure, bump PORTREVISION only
for powerpc64 to avoid all other users having to rebuild, too.)
PR: 239648