adjust to new "configure -nomake tools" semantics

this also reverts f47715f0a4 on the way, as this should make it
obsolete.

Change-Id: Ib54177b35f2378d7e6df497f546a8c4f19d00914
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
make use of qt_app.prf

Change-Id: Ibb906772daf7a6bc6d56874db0a4ced2cafa67a4
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>