autoconf: import gcc function attribute check macro

autoconf: import gcc function attribute check macro

Import Gabriele Svelto's AC_GCC_FUNC_ATTRIBUTE autoconf macro to check for
GCC-style function attributes.  This macro is part of the GNU Autoconf
Archive[1]. The imported file is distributed under an all-permissive license.

[1] https://www.gnu.org/software/autoconf-archive/

Change-Id: I64ccd00717fa9606a26aeeeea9030f4fb4877cf8
Reviewed-on: https://gerrit.openafs.org/12962
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>