gnulib, autoconf: update.

gnulib, autoconf: update.

* README-hacking (Updating a submodule): Give advice on how to
determine the versions of gnulib and autoconf to which we should
update.
(Release Procedure): Note that submodules should be updated.
* bootstrap.conf (gnulib_modules): Rename pipe to spawn-pipe as
suggested in updated gnulib NEWS.
* gnulib: Choose a stable snapshot according to advice in Bison's
README-hacking.
* po/POTFILES.in (lib/pipe.c): Rename to...
(lib/spawn-pipe.c): ... this.
* src/output.c: Update to include spawn-pipe.h.
* submodules/autoconf: Update to latest for improvement in m4.m4
that excludes M4 with buggy strstr.  The only other changes to
files that we use are copyright updates.
(cherry picked from commit a898435b25eca4869e8c49b2b0fb1f649b030ce7)

Conflicts:

	build-aux/.gitignore