I have put up a simple Qt project to github/harmattan that is probably the easiest way to show the changes needed to a project to get it compiling both on qt4 and qt5:

https://github.com/harmattan/helloqt5

It's based on the QtQuick project template created by QtCreator. The only thing missing is that *.qml files imports have to be modified manually when you switch between versions.

I'm running out of time so I'll be progressing more slowly from now on. Probably I'll start doing stuff on this again when device arrives.