setup.py: add "build_ext --cython=" option, which allows users to specify the path to the cython executable.

this also allows users to skip automatic cython compilation completely
by specifying an empty cython executable, e.g. by providing the
following setup.cfg:

,----[ setup.cfg ]
| [build_ext]
| cython=
`----

I use this to manage a copy of gevent inside another git repository
while at the same time making sure that core.c doesn't get
regenerated.