provide a generic named USE_DBUS option to allow to choose if to use it

per default ON on Linux & BSD systems off for other stuff: win/mac/android/haiku

via USE_DBUS=ON the user of the framework can override that

# shall we use DBus?
# enabled per default on Linux & BSD systems
set(USE_DBUS_DEFAULT OFF)
if(LINUX OR BSD)
    set(USE_DBUS_DEFAULT ON)
endif()
option(USE_DBUS "Build components using DBus" ${USE_DBUS_DEFAULT})
if(USE_DBUS)
    ...
endif()