Fix name of USE_SYSTEM_QT5 cmake var
This variable should be called USE_SYSTEM_QT5 not USE_SYSTEM_QT. Currently it is not possible to use the system's QT5 with USE_SYSTEM_QT=ON, it should be USE_SYSTEM_QT5=ON.
This variable should be called USE_SYSTEM_QT5 not USE_SYSTEM_QT. Currently it is not possible to use the system's QT5 with USE_SYSTEM_QT=ON, it should be USE_SYSTEM_QT5=ON.