Build: configure: put more effort on detecting Upstart support

When it's not explicitly required nor disabled, the support is
investigated using dbus-send to query its DBus-exposed "version"
property (existence of basic DBus support is attested first for
this case as well, and for special cases of such Upstart support
differing between the build and target machines whilst both
meet this basic DBus support, one can still pass --enable-upstart
explicitly).