Append "lib-tk" to PYTHONPATH if Tkinter extension is enabled

This will fix tcl tests on Linux. On MacOSX, the module Tkinter can
now be imported by there are still issue to import "_ctypes" module.
Merge branch 'add-lib-tk-to-pythonpath'

* add-lib-tk-to-pythonpath:
  Append "lib-tk" to PYTHONPATH if Tkinter extension is enabled