mesa*: Switch to meson
Upstream switched to meson for their build system so do the same as
it will be easier to find problems sooner than later.
This also switch to python3, python is only used for the build.
There is more thing enabled by default (or auto-enabled based on some
configuration) so we need to disable a lot more things than with autotools.
This should be a non functional change.
Reviewed by: zeising (x11@)
Differential Revision: https:/reviews.freebsd.org/D25092
Update the Mesa ports to 17.3.1 and switch to LLVM5 by default
Correct dependencies: remove the cruft added to mesa-dri in the previous
commit (those are mesa-libs deps), add a RUN_DEPENDS for libclc to clover
Clean up patches, remove one stale patch