I've seen than media-libs/allegro ebuild (and also the installation scripts of Allegro sources) don't install the useful "setup" utility, stored in /setup/ subdir of source tree. IMHO it's a necessary utility that permit to forge the allegro.cfg configuration file, needed in most cases, when the user try to play an allegro-based game. I attach a modified ebuild of allegro-4.2.0-r1 (actual stable version), that install the setup binary (and required data files) and make a desktop entry.
Created attachment 101910 [details] allegro-4.2.0-r1.ebuild
thanks