appveyor: Add support for meson as well as scons on windows

appveyor: Add support for meson as well as scons on windows

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>