Fix Windows CI

[ianfixes]

Set up Windows CI using Appveyor

@pruten to provide information on the build process

[ianfixes]

This is party done -- the CI runs -- but we are blocked until I can find a way to install SDL2 on the appveyor machines and have that installation be recognized by the build scripts.

[tsal]

As an interested party (not so much in Windows build, but in the repo) - how far of a stretch would it be to port the build to use visual studio 2019 + cmake? vcpkg is really good at installing frameworks and libraries pre-wired in appveyor to be available to the builds.

[ianfixes]

Sorry for my pandemic-related distraction on this, cmake is 100% what I'm interested in at the moment, not just for this but for the macemu projects as well.

My cmake skills are rusty, especially when it comes to files that are generated by an external program prior to build, and then used as sources. But I will welcome (& assist as I can) any cmake-based build progress.

[rakslice]

I'd be much obliged if you can take 5 minutes to look at #1 and see if that windows build is misconfigured in some obvious way

I mean, why is it configured to do what it's doing now? I don't know much about cygwin 64, or shoebill, but presumably if it's missing a header you make sure the package it would be in is in the list of packages you're installing?