Having discover handle .deb packages is useless in non-Debian based distros. Same for .rpm. Almost all the times someone clicks on a .deb or .rpm file in a distro with a different package format they expect the archive to be opened in an archive extractor. Having it open in discover is annoying -at the very least- and potentially dangerous (if, for some reason, the wrong PK backend is installed, Discover may try to install the package, which could wreck the system).
This patch makes handling of .deb and .rpm optional at build time.