mesa-7.5.2 installs libGL into "/usr/lib/opengl/xorg-x11/lib/", however, it installs /usr/lib/pkgconfig/gl.pc specifying "libdir=/usr/lib" This causes software which depends on GL to not include the correct -L flag for the linker, where builds fail for not being able to find libGL.
Probably invalid - eselect-opengl creates symlinks.