INCLUDEPATH += [path]
This is especially useful if like me, you add core code to a separate .lib project but you don;t want to use relative paths in your #include statements.
You can add as many of these INCLUDEPATH statements as you like.
Eventually I'd like to see support for this in the IDE. In fact, I'd go as far as saying the editor should expose lots more additional .pro functionality without the user having to edit these files themselves.