In the main menu of xfce4, synaptic is launched by pkexec-synaptic, which is a script invoking pkexec. pkexec is part of policykit and obtains permission from the policykit system. If policykit isn't installed, it is still possible to use gksu, but the menu launcher still invokes the non-existing pkexec.

On the other hand, synaptic doesn't depend on policykit or gksu; it only recommends them.

Is there a possibility that the packaging system be flexible enough to provide a menu launcher matching either policykit, gksu or the kde counterpart, depending on which one is installed?

A solution might be to use a link in /etc/alternatives.

Thanks.