An issue was reported on PyPy where a user had a ~/.local/lib/python2.7/site-packages directory that contained cpython specific libraries. We trakced it down to posix_user in sysconfig.py being set to an implementation-specific cpython path, but used by pypy.
This patch  against HEAD adds {implementation} and {implementation_lower} fields to _CONFIG_VARS, making sysconfig.py compatable with other python implementations. Perhaps valuable to jython as well, which would require modifying _get_implementation() accordingly