Fix documentation problems reported by Russ Boylan in

Fix documentation problems reported by Russ Boylan in
<http://lists.gnu.org/archive/html/bug-autoconf/2004-11/msg00056.html>,
along with some nearby cruft.
* doc/autoconf.texi (Libtool): Libtool can be used without
Automake (not without Autoconf).
(Introduction): Mention lists.gnu.org.
* BUGS: Don't mention bugs.gnu.org.
Remove mention of ancient libtool compatibility problem.
* NEWS: Mention that bugs.gnu.org is kaput.
* README: Likewise.  Mention where mailing list archives can be found.