macos: add support for MacOS 10.13

macos: add support for MacOS 10.13

This commit introduces the new set of changes / files required to
successfully build the OpenAFS source code on OS X 10.13 "High Sierra".

Reviewed-on: https://gerrit.openafs.org/12741
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 804c9cbf501d4ca91b69ad8fd6d64e49efa25a47)

Change-Id: I9abcccf8313c8ac075eb1edbd36cbaa565968b38
Reviewed-on: https://gerrit.openafs.org/12825
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>