Get real `site-packages`

build_ext: always import _CONFIG_VARS from distutils.sysconfig instead of sysconfig

README and CHANGES were renamed. Update MANIFEST.in to include them once again in the source distributions. Fixes #575.

otherwise `distutils.sysconfig.customize_compiler` does not configure OSX compiler for -dynamiclib

See https://github.com/pypa/setuptools/issues/571

Fix setuptools url and curl wants --location to follow redirects

This change also removes:

    sys.meta_path = [importer for importer in sys.meta_path if
                     importer.__class__.__module__ != 'pkg_resources.extern']

from _unload_pkg_resources.

These lines were removed from the version at: https://bootstrap.pypa.io/ez_setup.py

I don't know why the code there and here differs.