Please test media-libs/allegro-4.2.2 and media-libs/allegrogl-0.4.3. It looks like -sparc was added to allegro-4.1.16 in 2004. http://sources.gentoo.org/viewcvs.py/gentoo-x86/media-libs/allegro/allegro-4.1.16.ebuild?r1=1.1&r2=1.2 The problem should be fixed by now... Thanks.
Added back ~sparc for testing, and closing.