extra flags to setup.py

[John Hunter]

I am using distutils to distribute a python package.  I would like
some of the extension module builds to be optional, eg, to be built
only if the user types

  > python setup.py install --with-some-extra-module

But reading over the docs at
http://www.python.org/doc/current/dist/dist.html, I didn't see how to
add and use extra command like flags.

Thanks,
John Hunter