sphinx: set highlight_language to c

this is the default language used for displaying snippets in the code when no language is specified. defaults to python-ish, but it makes sense to default to c for this template

http://www.sphinx-doc.org/en/stable/config.html#confval-highlight_language

(although strictly speaking this is not a bugfix but an enhancement, I think it is small enough to include for 1.9.1)