Add QT version to environment

I came across a problem while trying to compile same project with different versions of QT, as I can't really tie specific QT version to specific build folder. It would be really nice if we could get another environment variable (like existing QTDIR for base QT folder location) for QT version - QTVER or something similar. This way we could use it in project properties for build/output folders like "\$(Configuration)\$(QTVER)\"