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.

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
 
@@ -200,6 +214,8 @@ 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