depend_prl would so far only ensure that the dependency existed, not
that it would be up to date. this is of only limited value, so make sure
that the dependencies are always updated.
as this is somewhat expensive (every dependency's makefile will be
entered as many times as it is used, plus one), provide an opt-out by
adding fast_depend_prl.
Task-number: QTBUG-29486
Change-Id: Ifa2e100bc4c269a403ab620927bfe5c7efe9a435
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
be1116fe90