I manually removed the dependancy from the xfree on the 3.7.6 version of the driver and it worked with xorg-x11-6.7. Some aditional stuff needed to be done thought: create a simbolic link from /usr/lib/opengl/ati/extensions to /usr/lib/opengl/xorg-x11/extensions. I don't know if the requirement for doing this is a bug with xorg-x11 or not. Anyway the driver works OK and every opengl application I tried is running fine. Reproducible: Always Steps to Reproduce:
Fixed in 3.7.6-r1
I just noticed that it might also be needed to add a symlink to the include directory on opengl/xorg-x11 at opengl/ati otherwise some packages may fail to compile (as I've just seen happening with games-strategy/scorched3d).
From the 3.7.6-r1 ebuild: dosym ../${X11_IMPLEM}/include ${ATI_ROOT}/include dosym ../${X11_IMPLEM}/extensions ${ATI_ROOT}/extensions