This patch does two things for being compatibile with
those Python which are built without threading support:
1. As the Python document and Shawn suggested, import dummy_threading
when the threading is not available.
2. Reserve the single threaded code and make it default.
In cases the --jobs does not work properly with dummy_threading,
we still have a safe fallback.
Change-Id: I40909ef8e9b5c22f315c0a1da9be38eed8b0a2dc