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 a898435b25)

Conflicts:

	build-aux/.gitignore

README-hacking

@@ -158,6 +158,20 @@ Register your changes.
 
         $ git checkin ...
 
+For a suggestion of what gnulib commit might be stable enough for a
+formal release, see the ChangeLog in the latest gnulib snapshot at:
+
+        http://erislabs.net/ianb/projects/gnulib/
+
+The autoconf files we use are currently:
+
+        m4/m4.m4
+        lib/m4sugar/m4sugar.m4
+        lib/m4sugar/foreach.m4
+
+These files don't change very often in autoconf, so it should be
+relatively straight-forward to examine the differences in order to
+decide whether to update.
 
 * Test suite
 
@@ -203,6 +217,8 @@ release:
 
 * Release Procedure
 
+** Update the submodules.  See above.
+
 ** Try to get the *.pot files to the Translation Project at least one
 week before a stable release, to give them time to translate them.
 Before generating the *.pot files, make sure that po/POTFILES.in and