fix building against non-installed qt5 shadow builds

the global qdoc configs may live in the source dir, but the indexes are
always in the build dir.

this will obviously only work for a non-prefix (developer) build of qt,
as a non-installed prefix build is not usable from the outside.

Task-number: QTCREATORBUG-11919
Change-Id: I268af2f075a098ac5fe788a2f85c754dc6e4e8b0
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>