macos: add support for MacOS 10.15

macos: add support for MacOS 10.15

This commit introduces the new set of changes / files required to
successfully build the OpenAFS source code on OS X 10.15 "Catalina".

Reviewed-on: https://gerrit.openafs.org/13668
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 93815caabc92acc6edc62b72805b44d2e46748cf)

Change-Id: Ia1fb98dd59d7b0ddad9c16c04b823623e07dd498
Reviewed-on: https://gerrit.openafs.org/14033
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>