Clean up thread configuration.

Clean up thread configuration.
* qthreads.m4: New file, which knows how to configure the qthreads
library.
* configure.in: Replace all thread package selection code.  Do the
--with-threads argument processing here.  Enable the appropriate
thread interface files in libguile.  Remove all qthreads
configuration code; call QTHREADS_CONFIGURE instead.  Set
GUILE_LIBS using the info provided by QTHREADS_CONFIGURE.
* threads.m4: Removed; not used any more.
* Makefile.am (aclocal_DATA): Mention qthreads.m4, not threads.m4.
* Makefile.in, aclocal.m4, configure: Rgnrtd.  (Sv th vwls!)
Note that these were regenerated with the tools available from
Cygnus's source tree, which have patches not available to the
general public.  I'm not sure this was a good idea; feel free to
revert them to the latest released versions of the tools.