Fix documentation installation for qmake.

- Do not build docs when installation is requested.
- Introduce a nicely named target ("install_docs").

Change-Id: Ia75503e55adec84fe12a6adfe3cd6acbdd545e34
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>