sync with qmake. this is a no-op in qtc.

Change-Id: Id5158d7e272fdee4f4a041fb7c828295a0a86684
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
(cherry picked from qtbase/d135a311a34e693d761a3607663b0879be9bbf65)