libselinux: query for python site-packages dir directly

With the bump to version 2.6, the following commit needs
to be taken into consideration for overloading paths.
https://github.com/SELinuxProject/selinux/commit/8162f10e670da963eb65ccf1e7de69ea85aba30d

The PYLIBVER is no longer used and the PYTHONLIBDIR is
renamed to PYSITEDIR with slightly different pathing.

More details can be found in the issue ticket which was
marked as a non-issue after analysis that a Buildroot fix
was the resolution.
https://github.com/SELinuxProject/selinux/issues/51



Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>