qmake: make more use of ProString built-ins

saves some noisy toQString() uses.

Change-Id: I62a9e2725c4baabac311124d19c7d8b40f54c8f7
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
(cherry picked from qtbase/ce5e6876d4a191087969134e489db99cf167ca69)
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>