Add breathe extension configurations to the state.

def _insert_breathe_configs(c, *, project_name, doxygen_xml_dirname):
    """Add breathe extension configurations to the state.
    """
    if doxygen_xml_dirname is not None:
        c['breathe_projects'] = {project_name: doxygen_xml_dirname}
        c['breathe_default_project'] = project_name
    return c