ENH: Use the Mac -framework for Tk if it is available.

Modules/FindTCL.cmake

@@ -143,6 +143,9 @@ IF(APPLE)
   IF("${TCL_INCLUDE_PATH}" MATCHES "Tcl\\.framework")
     SET(TCL_LIBRARY "")
   ENDIF("${TCL_INCLUDE_PATH}" MATCHES "Tcl\\.framework")
+  IF("${TK_INCLUDE_PATH}" MATCHES "Tk\\.framework")
+    SET(TK_LIBRARY "")
+  ENDIF("${TK_INCLUDE_PATH}" MATCHES "Tk\\.framework")
   IF(TCL_HAVE_FRAMEWORK)
     IF(NOT TCL_LIBRARY)
       SET (TCL_LIBRARY "-framework Tcl" CACHE FILEPATH "Tcl Framework" FORCE)