This job uses the vs2017 backend of meson (msbuild) as opposed to the
ninja backend used on MacOS and Linux.

v7: - rebase on master
    - remove llvm (we'll add that back later)
    - remove cygwin (we'll add that back later too)
v6: - rebase on master, including the addition of cygwin
    - consolidate 3 appveyor patches into this one patch
v5  - use the new b_vscrt option instead of manually specifying the crt
v4: - rebase on python3 generators
    - cache meson wraps
    - Build x86 instead of x86_64, since that's what the pre-built LLVM
      is
    - update to vs2017 from vs2015
    - set the default-library to static
    - use the new vscrt override
    - add the /m switch to msbuild to make the build somewhat faster

Acked-by: Kristian H. Kristensen <hoegsberg@google.com>