Allow overriding SETUPTOOLS_USE_DISTUTILS

[Gu1nness]

Necessary for the package to build on debian.
Cf pypa/setuptools#2956

[jborean93]

This is not something we want to add into the setup.py file. It's an environment specific setting and really just a workaround for a bug that either Debian or pip need to fix on their end. In any case setuptools actually defaults to local by default which is where the problem stems. Using stdlib will only work until Python removes distutils in the stdlib which is coming in a future version.