More meson for windows

Dylan Baker requested to merge dbaker/mesa:meson-windows-p2 into master

This is the second batch of patches for using meson on windows. This is more complete and has more recent and unreviewed work than any of the previous batches. I've also attempted to get appveyor and gitlab-ci running, we'll see how many iterations it takes to get that all working.

Most notably this does not include support for using LLVM. Those patches have produces the most rebase headaches for me (all of them, in fact) and I want to land all of this, and then get LLVM wired up after building softpipe works. This means that we can't build SWR or LLVMPipe yet, just softpipe.

Edited by Dylan Baker

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