xmame-0.103 was released today, to build it needs just a version bump of the xmame-0.102 ebuild. It works for me (tested on Amd64).
Created attachment 77376 [details] xmame-0.103 ebuild bump
please go read games.gentoo.org
(In reply to comment #2) > please go read games.gentoo.org > oops, sorry for the bump request