pkg-python: also pass PYTHONPATH when building distutils packages

With the upcoming bump of Python 2.x, it will become important that
the PYTHONPATH is passed whenever we build third-party packages, be
they using the distutils build mechanism, or the setuptools build
mechanism. This is because passing PYTHONPATH is what will allow
Python to find a special Python module that contains all the
compiler/library/headers definitions that are relevant when
cross-compiling.

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