Use autoconf-archive m4 from src/external
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.
Change-Id: I8acfe4df7b9a22d9b9e69004c3438034a2dacadb
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>