Switch to using the m4 macros from autoconf-archive in our
src/external mechanism, instead of manually-copied versions in src/cf.
The src/external copy of ax_gcc_func_attribute.m4 is identical to the
existing copy in src/cf, so that should incur no changes. There are
also a few new macros pulled in, but they are currently unused.

Increase our AC_PREREQ in configure.ac to 2.64, to match the AC_PREREQ
in some of the new files.

Reviewed-on: https://gerrit.openafs.org/14135


Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit ca847ddf)

Change-Id: Ifa43b3869e426fada5bd925b0ae002a0f6436232
Reviewed-on: https://gerrit.openafs.org/14944


Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>