libxcb: fix path to Python modules

When building libxcb, the variable XCBPROTO_XCBPYTHONDIR must point to
the location where the Python modules needed to run the c_client.py
program are installed. The path
$(STAGING_DIR)/usr/lib/python2.6/site-packages was hardcoded. However,
it doesn't work when the version of Python installed on the host is
Python 2.5.

Therefore, add a little bit of magic to compute the host Python
version.

We also verify that Python is available on the host, as we don't build
it in Buildroot.

Fixes bug #1531.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>