Cython build_ext support broken when using setuptools/distribute
When a Cython Extension is added to the setup.py, building the extension does not work as it does not process the pyx file if the pyrex package is not installed.
This seems to be an issue existing for a long time. See for example : http://www.velocityreviews.com/forums/t693861-cython-setuptools-not-working-with-pyx-only-with-c-files.html
This can be easily fixed with the given patch.