build: support autobuild

build: support autobuild

* cfg.mk (gnulib-update): Add autobuild.m4.
* configure.ac (AB_INIT): Output autobuild header.
* m4/autobuild.m4: New file, from gnulib.
* build-aux/config.guess: Resync from upstream.
* build-aux/config.sub: Likewise.
* build-aux/texinfo.tex: Likewise.
* doc/fdl.texi: Likewise.
* doc/gnu-oids.texi: Likewise.
* doc/make-stds.texi: Likewise.
* doc/standards.texi: Likewise.
* build-aux/gendocs.sh: Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>