Context: 

 - Providing french translation to docs.python.rg/fr/: http://bugs.python.org/issue26546
 - Simplifying my first proposition of docsbuild-scripts: https://github.com/python/docsbuild-scripts/pull/1

Goals:

 - Simplify my modifications of docsbuild-scripts to make it more robust
 - Avoid executing external (~untrusted) Makefile on docs.python.org servers

To achieve this, I need the ``Doc/Makefile`` to be sufficient to build internationalized versions by passing sphinx options to it, I'd like to call, typically:

    make autobuild-stable SPHINXOPTS='-D language=fr -D locale_dirs=./locale/'

Which work if `Doc/Makefile` don't erase but append to the `SPHINXOPTS` variable. Which is done by `allow_sphinxopts.diff`.

We may simplify it further by adding `locale_dirs` to `Doc/conf.py` which does not break the english build, but I'm not sure if it's of any interest.

We may abstract it further by accepting a new parameter like SPHINXLANG='fr', with a default of 'en', but in every case I think it's a good thing to allow passing arbitrary SPHINXOPTS so let's start with this?