I did install mesa from sources, git master...
But not manually, with a gentoo ebuild: maybe there's something wrong with it. I will try to get more info.
Have you compiled mesa with VIDEO_CARDS="radeon" set and xorg-server with "dri" USE flag enabled?
Maybe using xorg-server from git is not necessary but it won't hurt either - you may need to fix osdeps.h file in its's source code to make it compile on Gentoo box.