Tried to build mesa-13.0.2 with the vulkan USE flag enabled, and it failed with an expected `configure: error: LLVM 3.9.0 or newer is required for radv`. Bug title explains it all.
Fixed with commit be6e5d346a1544500dcdae39e28d1f1a057a1717 Author: Matt Turner <mattst88@gentoo.org> Date: Wed Jan 25 14:52:02 2017 -0800 media-libs/mesa: Depend on >=llvm-3.9.0 if building radv.