Use AM_PATH_PYTHON and python-config to detect Python configuration

Use AM_PATH_PYTHON and python-config to detect Python configuration

Using AM_PATH_PYTHON solves the site-packages directory problem. At least
in Ubuntu with Python 2.6 and later site-packages is renamed to dist-packages
and site-packages is not part of sys.path anymore. So installing the libvirt
Python bindings to site-packages renders them unusable, because they can be
imported from there without manually including site-packages into sys.path.

AM_PATH_PYTHON detects the correct site-packages/dist-packages directory.

python-config --includes gives the correct include path for the Python header
files. The old probing code stays there as fallback mechanism.

* configure.in: use AM_PATH_PYTHON and python-config
* python/Makefile.am: remove -I because PYTHON_INCLUDES contains it now