don't pass qmake configure arguments to sub-projects

the arguments after '--' are by definition meant only for the top-level
project, as that's where configure is invoked from. passing them to
sub-projects just adds noise to the make output and misleads users.

note that this specifically does not support qmake -r, which will break
if the subprojects rely on the arguments being absent. this isn't a
problem, because the qt build doesn't support qmake -r anyway.

Change-Id: I7ecff6212ce3137526005fc324a4a7ae45e3345e
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
(cherry picked from qtbase/34cc41d8a17e6e30f01f22c5d382c28d49ae37e1)
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>