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.

po/.gitignore

@@ -1,3 +1,4 @@
+
 /*.gmo
 /*.po
 /*.po~
@@ -20,3 +21,4 @@
 /remove-potcdate.sed
 /remove-potcdate.sin
 /stamp-po
+

po/POTFILES.in

@@ -22,7 +22,7 @@ lib/bitset_stats.c
 lib/error.c
 lib/getopt.c
 lib/obstack.c
-lib/pipe.c
+lib/spawn-pipe.c
 lib/quotearg.c
 lib/timevar.c
 lib/w32spawn.h