cf: Make local copy of ax_gcc_func_attribute.m4
Make a local copy of ax_gcc_func_attribute from autoconf-archive. This
is needed in order to fix a bug in the detection of the fallthrough
attribute.
Remove ax_gcc_func_attribute.m4 from src/external/autoconf-archive/m4.
Update LICENSE file to point to the local copy in src/cf.
Change-Id: I6c4244d2cd4edab4262c1820435c00419d85303b
Reviewed-on: https://gerrit.openafs.org/14272
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>