[Cmake] suggestion for FindTclsh

[Wheeler, Fred (Research)]

I think that this section from FindTclsh.cmake (indentation added)

  IF(WIN32)
    IF(UNIX)
      FIND_PROGRAM(TCL_TCLSH cygtclsh80)
    ENDIF(UNIX)
  ENDIF(WIN32)

should be changed to this

  IF(CYGWIN)
    FIND_PROGRAM(TCL_TCLSH cygtclsh80)
  ENDIF(CYGWIN)

I have found it convenient and quite helpful to unset WIN32 to compile VXL in Cygwin.  One
side-effect is that this prevents cygtclsh80 from being found using the above, since WIN32 is unset.
I realize that unsetting WIN32 may be unusual.  Still, I think the change suggested makes things more
readable and makes more intuitive sense.  (WIN32 seems to imply that the Windows api is available.
Same for UNIX.  cygtclsh80 is not in the intersection of the windows and Unix apis.)

Just a suggestion.  Take it or leave it.

Thanks,
Fred Wheeler