this avoids that an empty rebuild after a complete build re-runs qmake
everywhere again.

according to 1f83f0cf

 this is the behavior i originally intended,
but somehow it got lost when switching to the new interpreter.

Change-Id: Id5158d7e272fdee4f4a041fb7c828295a0a86684
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>