If a project has DESTDIR and TARGET set to fixed values, then the
target paths conflict when doing debug_and_release builds.
With this change we're detecting this situation and yield a warning.
Fixes: QTBUG-2736
Change-Id: Ib163db3463322792ab9fa5b997285ac9fc9819ab
Reviewed-by: Kai Koehne <kai.koehne@qt.io>