Use PYTHONDONTWRITEBYTECODE=1 to prevent python from creating .pyc files
in the source tree. Python 3.2 has a __pycache__ scheme [1], but before
that the only alternative seems to be to copy the source files to the
build tree to ensure the .pyc files are created there too. Just prevent
.pyc file generation for simplicity.

Considering the small amount of python code to compile (assuming sphinx
itself has .pyc around), the impact on build is neglible.

[1] http://stackoverflow.com/questions/3522079/changing-the-directory-where-pyc-files-are-created

References: http://lkml.kernel.org/r/CAMuHMdVxqpH7-9XJ+YE_pgoA+-fe0969cSkOehYh3uubYcrhZA@mail.gmail.comReported-by: Geert Uytterhoeven <geert@linux-m68k.org>
References: http://lkml.kernel.org/r/1485816692.2900.17.camel@decadent.org.ukReported-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>