build: build deb/rpm independently on config/extensions in the host system Reverts 5aac617a028d and replaces it with a more general solution. - works for both rpm and deb - sidesteps eventual problems with local extensions that have nothing to do with the build process (hg-git, for example, fails with version 4.4 because dedab036215d removed peerrepository, and hg-git still uses it as of 0.8.9)