Log Message:
cad/occt: Find tcl under BUILDLINK_DIR.tcl, not BUILDLINK_PREFIX.tcl.

Likewise tk.  Needed for when we hide PREFIX from cmake.  Will
resolve issues like:

CMake Error at CMakeLists.txt:875 (message):
  Could not find libraries of used third-party products:
  3RDPARTY_TCL_LIBRARY_DIR 3RDPARTY_TK_LIBRARY_DIR