Current mesa git requires a newer version of libdrm and the ebuild does not reflect this at the moment. Here are the exact libdrm requirements: https://cgit.freedesktop.org/mesa/mesa/tree/configure.ac#n75
commit f8a421a83db456be711fa50a7e9715eea3137d96 (HEAD -> master) Author: Matt Turner <mattst88@gentoo.org> Date: Tue Jul 18 17:16:02 2017 -0700 media-libs/mesa: Require libdrm-2.4.81