http://project-builder.org/Project-Builder.org aims to provide support to the "Continuous Packaging" approach. The mantra is "Package early, package often".
Project-Builder.org is a tool that helps you build packages for your application (provided as tar file, or through a configuration management system such as CVS, Subversion, Mercurial, or Git). It is able to generate build package skeleton for your project, and with simple configuration files to generate for up to 50 different tuples of distributions (name, version, architecture), including Fedora, Mandriva, OpenSuSE, Debian, Ubuntu, Gentoo, Slackware.
Ich habe einen Vortrag darüber auf dem Linuxtag 2010 besucht.Project-Builder.org is a GPL tool to help you build packages of projects for multiple operating systems
Daneben gibt es ja noch den Build Service von SuSE. Allerdings weiß ich nicht, in wie weit und gut der für Slackware taugt.