support USE_DBUS option to enable/disable dbus use
default on for Linux and BSD systems
The old code was relying on
if(TARGET Qt6::DBus)
and
#ifdef QT_DBUS_LIB
=> that will still enable the DBus code pathr the QtDBus stuff is around for other deps.
Now we use a dedicated variable and define for that.
Edited by Christoph Cullmann