Install under lib/ or lib64/

[jesper-friis]

On RedHad-based linux distributions on x86_64 architectures, Python packages with C extensions (like DLite) are supposed to be installed under lib64/ instead of lib/. But DLite installed from cmake with default settings dies not follow this convension. The same is the case for old versions of pip. This may create confusion, especially if you are in a Python environment and first install dlite with pip and then with cmake.

Tasks to make it easy to resolve these cases add a cmake option for whether to use lib/ or lib64/, with a sensible default obtained by asking Python.