add `PYTHON_LIBDIR` constant and set it equal to `distutils.sysconfig.get_python_lib(prefix='')`, cfr https://github.com/hpcugent/easybuild-easyblocks/pull/141

the `OS_NAME` and `OS_VERSION` constants that were merged in with https://github.com/hpcugent/easybuild-framework/pull/547 will also need to be aded here