[crossfire] pthreads issue on Slackware 12.2

[Kevin Bulgrien]

See:

  https://sourceforge.net/tracker/index.php?func=detail&aid=2850517&group_id=13833&atid=113833

I have made an attempt to redo configure.ac and various client Makefile.am
files to try to bone up pthread checking and support in the clients.  Not
being an autoconf/make guru I had to resort to ripping off stuff.  I found
the ax_pthread m4 macro and made use of it.

I posted a patch to the above tracker (ax_pthread.patch).  It works for me.
I would appreciate some feedback on whether it works for some other
platforms - especially Slackware.  I could try to test myself, but it will
take a lot of effort to d/l an installer and figure out how to set up a VM
and development enviroment.  Please save me this grief.  It already took a
lot of hours to come up with the patch.

It appears to me that metaserver 1 is required and that only metaserver 2 is
optional.  Since both metaservers reference pthreads, I take this to mean
that we cannot build the clients without pthreads.  In keeping with this,
the client will not longer report it will build the clients if pthreads are
not found.

Feel free to chime in with comments if anything I said or did seems broken in
some way.

No feedback... then I'll commit anyway in a few days...

Kevin