Now the QT4 version of qmake can be explicitly selected by passing --buildsystem...

Now the QT4 version of qmake can be explicitly selected by passing --buildsystem=qmake_qt4. Closes: #566840

There is that build system option patch that I suckily never applied,
and could be used here.. but this is at its core a different build system,
and so handling it as such makes the most sense.

It may make sense to have a qmake_qt3 build system, but perhaps QT 3
will instead just go away. I considered just waiting, but this is an easy
fix. ;)