[CMake] Enhanced FindTCL.cmake

[Clifford Yapp]

In the course of my work converting BRL-CAD to CMake, I have had to
expand quite a bit on the standard FindTCL.cmake included with CMake.
Among other issues, BRL-CAD needs to be able to:

1) detect threaded vs. non-threaded tcl
2) determine if Tk is built for X11 or Aqua graphics systems (OSX)
3) specify version requirements (reject anything older than Tcl 8.5)
4) sort out multiple installs of Tcl/Tk on one system (OSX is the
common case, but potentially other systems too)

After much muddling around, I have (I believe) come up with something
that is capable of performing these and other tasks.  I would like to
ask if it could be folded back into the default CMake, but seeing as I
wrote it while still getting the hang of CMake there are probably a
number of cases were suboptimal techniques were used.  Would there be
anyone interested in doing a review of this new FindTCL.cmake with an
eye towards getting it to a state where the official CMake devs would
accept it?  (I have just attempted to switch to using VERSION_LESS and
friends instead of a custom version comparison module, so some testing
to make sure the new logic is right would also be a help.)

The script is rather long to post here, so I've put it in the bug database:

http://public.kitware.com/Bug/view.php?id=11630

Cheers,
CY