autoconf: fix detection for fallthrough attribute
Due to bug <https://savannah.gnu.org/patch/?9949>,
ax_gcc_func_attribute.m4 fails to properly detect __attribute__((fallthrough))
in clang. Until this is fixed in autoconf-archive upstream, fix our
local copy of ax_gcc_func_attribute.m4, so we can detect
__attribute__((fallthrough)) to make --enable-checking work with clang.
Change-Id: I80a4557384f8e1438344e48bfe722e20c8773882
Reviewed-on: https://gerrit.openafs.org/14273
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>