build_requires support

In bug #110133 (setup.py needs code from another egg to build) the case was discussed where a setup.py of a package could not proceed because other eggs were expected to be on the python path. A mention of build_requires was made as a possible solution, though in that this would not work, as I had no control over the other package's setup.py.

This time I'm writing a package myself so I could set a build_requires to mention that cython needs to be installed before building (http://www.cython.org/). There does not seem to be any response to using this command, however. Is this correct? I can find no clear documentation on this command - the only time it comes up is in the context of generating RPMs.

Here's my setup.py:

from setuptools import setup # tried with plain distutils too
from distutils.extension import Extension

from Cython.Distutils import build_ext

setup(
    name = "unbound",
    package_dir = {'': 'src'},
    packages = ['unbound'],
    ext_modules=[
    Extension('unbound.ub', ['src/unbound/ub.pyx', 'src/unbound/unbound.c']),
    ],
    headers=['src/unbound/unbound.h'],
    cmdclass = {'build_ext': build_ext},
    build_requires=['cython'],
    install_requires=['cython'],
    )

This feature, or the feature discussed in #110133 therefore appears to be needed to let buildout install packages that need special tools (such as cython, a fork of pyrex) to let themselves be installed.