cerbero: Support XCode Python 3 on macOS Big Sur
cerbero: Support XCode Python 3 on macOS Big Sur

python.org's official Python 3 release does not work on Big Sur, so
the only option at present is to use the Python 3 that ships with
XCode. Unfortunately, that doesn't ship with python3-config right now
so you need to build with `-v nogi`.

XCode's Python 3 defaults to a framework path for sysconfig purelib:
/Library/Python/3.8/site-packages but setuptools still uses the posix
path: /lib/python3.8/site-packages. This also means we'd have
inconsistent paths based on your macOS version.

The second issue is that on Windows the purelib path is
\Lib\site-packages which is not version-specific but it *is* the
default for setuptools, so let's consistently add both.

Part-of: <https://gitlab.freedesktop.org/gstreamer/cerbero/-/merge_requests/626>

GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,nirbheek/cerbero,nirbheek/cerbero,GStreamer/cerbero