xbmc-9999 fails to compile against libav (and libpostproc). ffmpeg-0.10.x is required. See here for more information: http://trac.xbmc.org/ticket/13096
Sorry, I don't understand, why this bug is resolved upstream. To quote the XBMC-dev: "based on that i think i'm not lying when i state: We are not compatible with libav atm." The ebuild let you compile xbmc-9999 with libav and libpostproc but this won't work. Maybe the XBMC-devs will change this one day (when libav releases a new version eventuelly) but atm they use api-fuctions of ffmpeg that are not available in libav. I think, the dependency >=ffmpeg-0.10 is required or you make an use-flag ffmpeg to toggle between interal and external ffmpeg (see here: https://bugs.gentoo.org/show_bug.cgi?id=275844)
