[CMake] Enhanced FindTCL.cmake

[Clifford Yapp]

Hmm... - on second thought perhaps this isn't a bug tracker issue?  I
thought since it was a change to an existing module instead of a new
module these Tcl/Tk improvements (and the opengl suggestion) were
suitable for reporting, but perhaps not - if not my apologies.  If the
bug needs to be closed I can re-post the file somewhere else.

Having (rather belatedly) spotted the readme.txt file in the Modules
directory, I see some things that I need to change to map to the
conventions of the readme (particularly with respect to version number
naming conventions).  I don't think the find_package versioning logic
by default has quite everything I need (e.g., compatibility is not
enough if you don't trust the package's compatibility numbering and
need at least version major.minor.patch for some feature but not
greater than major.minor.patch2 for compatibility reasons - hence the
addition of the MIN and MAX version numbers will still be needed), but
certainly it can be made to operate in line with the conventions as
far as they apply.

Looking back at the old FindTCL I see I inherited at least a couple of
the nomenclatures from there - for example:

TCL_INCLUDE_PATH and TK_INCLUDE_PATH are present but TCL_INCLUDE_DIRS is not

and TCL_TCLSH is an executable but does not use the _EXECUTABLE suffix.

I don't see a maintainer for FindTCL at
http://www.cmake.org/Wiki/CMake:Module_Maintainers so I'm not sure who
to ask - a strict reading of readme.txt seems to suggest that

TCL_TCLSH -> TCL_TCLSH_EXECUTABLE
TK_WISH -> TCL_WISH_EXECUTABLE
TK_LIBRARY -> TCL_TK_LIBRARY
etc.

while setting all of the old ones as well for compatibility but
marking them as deprecated. Would this be the desired way forward or
was a decision made in the past to go with the less verbose variable
names currently in CMake's FindTCL.cmake?

Cheers,
CY