[bug#33716] [PATCH staging] gnu: mesa: Update to 18.3.1.

[Rutger Helling]

Hey Guix,

Here's a Mesa update for the just released 18.3.1 version. Not sure
whether or not I'm too late for the staging branch. If I am this can
wait until the next staging branch.

Two things to note:
* While Mesa 18.3.x still builds with LLVM 6 the upcoming 19.0.x series
  doesn't anymore. For that and performance reasons I decided to add a
  address@hidden package for Mesa. I don't know enough about the LLVM stack
  to upgrade it entirely, so this is kind of a workaround.
* Since Mesa 18.3 works with Python 3 I've replaced the Python 2
  dependencies with Python 3 ones.

0001-gnu-Add-llvm-7.0.0.patch
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0002-gnu-mesa-Update-to-18.3.1.patch
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pgpJlnvhHyTBh.pgp
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