handle Sphinx's fix for C domain name conflicts

Commit [b2a0904](https://bitbucket.org/birkenfeld/sphinx/commits/b2a090415961d5efb00dabe3614e978fb853f0be) added a "c." prefix to all targets in the C domain. As a result, links to breathe-generated targets [don't work](http://breathe.readthedocs.org/en/latest/domains.html) because the anchors and IDs don't match. This change allows links to breathe-generated targets to work again.