Detect Python packages directory on configure

The Python interpreter on Debian is configurated to use
...lib/python.../dist-packages instead of
.../lib/python.../site-packages to look for extra modules.

python_modules.m4 performs a runtime python check of the
sys.path variable to check wether the selected interpreter is
configurated to use site-packages vs dist-packages.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>