Add a mechanism for using autoconf to detect the quirks of Unix systems

rather than relying on the user to edit the Makefile.  Makefile.gtk
still works as well as it ever did, but now we get a Makefile.in alongside
it.  mkunxarc.sh now relies on autoconf and friends to build the configure
script for the Unix source distribution.

git-svn-id: svn://svn.tartarus.org/sgt/putty@5673 cda61777-01e9-0310-a592-d414129be87e