Provide a proper fix for a problem r2384 tried to work around

Look first for sphinx-build, then for sphinx-build-2.6. As part
of this change, it is now required to run make.py with ost as
we make use of the settings.Locate() function

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2390 5a81b35b-ba03-0410-adc8-b2c5c5119f08