[media-libs/mesa] fork from tree, fix llvm dependency (make it a RDEPEND)

[   124.451] (EE) AIGLX error: vboxvideo does not export required DRI extension
[   124.451] (EE) AIGLX: reverting to software rendering
[   124.452] (EE) AIGLX error: dlopen of /usr/lib/dri/swrast_dri.so failed (libLLVM-3.3.so: cannot open shared object file: No such file or directory)
[   124.452] (EE) GLX: could not load software renderer

See Gentoo bug #488216.