Here his an ebuild that uses the mmx patch from http://members.jcom.home.ne.jp/jacobi/linux/xmms.html if MMX is enabled in the USE variable.
Created attachment 927 [details] xmms-1.2.7-r5.ebuild
Thanks for that. the -r7 ebuild in portage makes use of the flag if "3dnow" or "mmx" are set in USE,