in non-verbose mode, suppress qmake output as well

otherwise missing pkg-config requirements, etc. will be visible.

Change-Id: I5a5c0f35cf75e1fcc09ddbc7fcc54cef6bfec8b5
Reviewed-by: Qt Doc Bot <qt_docbot@qt-project.org>
Reviewed-by: J-P Nurmi <j-p.nurmi@nokia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>