Mesa update

[coypu%sdf.org@localhost]

hi current-users,

-current is now going to use mesa 18.3.4, and on x86, LLVM for radeon
and software acceleration. It's faster and supports more modern OpenGL
functionality. Software raster on x86 is now done using the faster
llvmpipe.
(Thanks to mrg@ and joerg@).

This will increase your build times dramatically if you build Xorg on
x86, from building LLVM libraries.

If you would like to do an update build, you will likely have to remove
many directories in OBJDIR/external/mit/xorg/lib/*.
I didn't test this, sorry.

Let me know if there are any situations for which this fails to work. We
got really good testing of things before committing it so I don't expect
much trouble.