Document minimum supported compiler versions

Document minimum supported compiler versions

Pick some fairly old versions of clang and gcc and document them
as the minimum supported version.  This will let us make assumptions
about compiler features that are available when using those compilers.

Change-Id: Ibb8df72c9b12cc7adff39ece9708a428975ba703
Reviewed-on: https://gerrit.openafs.org/12331
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>