python: Generate files into build directory

* lang/python/setup.py.in: Generate files within BuildExtFirstHack
adjust build flags at this point instead of global.
* lang/python/Makefile.am: Remove logic of separate source directory per
python version in favor of build directory.
* lang/python/tests/run-tests.py: Adjust build directory location.

Generate files into build directory, leaving the source directory clean.
Use the same source directory for multiple python version build. Result
of 'prepare' target is a standard distutil layout that can be used
easily by downstream to build all python targets in-place.

• Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>