This way a proper separation between Python installs can be achieved.
With no PYTHON_SITEDIR, installation paths are exactly the same as
before this change.
In practise, passing nonstandard path will break Entropy but so it is
also with the currently available LIBDIR. This is a concern of a future
improvement (at least the PYTHON_SITEDIR path, not necessarily LIBDIR as
PYTHON_SITEDIR *will* be different if it's installed in Python specific
directories).
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