If we use the Google Custom Search API, we do not need to keep the
generated searchindex.js file anymore. We also can safely remove
the _sources directory for good.
Since these get generated during each Sphinx build, additional
steps have been added to the Makefile to get rid of them. They
are also added to .gitignore to avoid commiting them by accident.
On MacOS X there is no sha1sum. So to simplify doc generation on those systems
use a Python script instead to generate a unique string from the repository
path.
