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.

src/output.c

@@ -24,8 +24,8 @@
 #include <configmake.h>
 #include <error.h>
 #include <get-errno.h>
-#include <pipe.h>
 #include <quotearg.h>
+#include <spawn-pipe.h>
 #include <timevar.h>
 #include <wait-process.h>