The games-emulation/xmame ebuild does not contain a parts in the build and install sections for creating the opengl-enabled binary (xmame.xgl, if the "opengl" USE flag is enabled) along with the other xmame binaries (xmame.x11, xmame.sdl, xmame.svgalib, etc...). Please see attached patch for reference - it has only been tested in the sense that I was able to make it compile/install without error and run it (seemingly OK) afterwards (proving that it CAN work, but will obviously need more in-depth testing). A short discussion on this topic can be found in the gentoo forums at http://forums.gentoo.org/viewtopic-t-7351.html
Created attachment 74032 [details, diff] OpenGL patch to the xmame-0.102 ebuild Patch to xmame-0.102 ebuild to include building of opengl-enabled xmame (xmame.xgl).
it's built in now. read the docs.
Comment on attachment 74032 [details, diff] OpenGL patch to the xmame-0.102 ebuild