dont add CUBES_DIR to cubes search path if it doesn't exists
Document: cubicweb-docTitle: CubicWeb documentationAuthor: LogilabAbstract: Some base documentation for CubicWeb users and developpersSection: Apps/ProgrammingFormat: HTMLIndex: /usr/share/doc/cubicweb-documentation/index.htmlFiles: /usr/share/doc/cubicweb-documentation/*.html