[libqtsparql] Avoid trampling on QT_{SOURCE,BUILD}_TREE.

Doing this prevents Qt 4's qt_config.prf from correctly locating qconfig.pri,
thus QT_MAJOR_VERSION not being defined. While this is fixed in Qt 5, it's still
probably not a good idea to trample on things we don't own.
[libqtsparql] Build with as many jobs as we can.