macos: add support for MacOS 10.12

macos: add support for MacOS 10.12

This commit introduces the new set of changes / files required to
successfully build the OpenAFS source code on OS X 10.12 "Sierra".

Reviewed-on: https://gerrit.openafs.org/12419
Reviewed-by: Joe Gorse <jhgorse@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 0bdf750a962a81b9b2e61387d7a3340dabb13395)

Change-Id: I693878212ec7cf00c2c8376ed5a1bab1198594b3
Reviewed-on: https://gerrit.openafs.org/12431
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Tested-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>