emerge -av mesa

These are the packages that would be merged, in order:

Calculating dependencies -
!!! All ebuilds that could satisfy ">=x11-proto/glproto-1.4.8" have been masked.
!!! One of the following masked packages is required to complete your request:
- x11-proto/glproto-1.4.8 (masked by: package.mask)

For more information, see MASKED PACKAGES section in the emerge man page or
refer to the Gentoo Handbook.
(dependency required by "media-libs/mesa-6.5.1-r4" [ebuild])