Add windows CI

[ctreffs]

Description

This PR aims to add a Windows CI job.
It should function analogous to :

• https://github.com/SwiftDocOrg/Markup/blob/4bfa96fbfd04edca5137773bb6a8be545693bc04/.github/workflows/ci.yml#L50
• https://github.com/tensorflow/swift-apis/blob/f51ee4618d652a2419e998bf9418ad80bda67454/.github/workflows/windows.yml#L33
• https://github.com/SwiftDocOrg/swift-doc/blob/f935ebfe524a0ff27bda07dadc3662e3e45b5125/.github/workflows/ci.yml#L133
• https://github.com/google/swift-benchmark/blob/main/.github/workflows/tests_windows.yaml
• pkg-config Windows mesonbuild/meson#4029

Checklist

• I've read the Contribution Guidelines
• I've followed the coding style of the rest of the project.
• I've verified that my change does not break any existing tests or introduce unexpected benchmark regressions.

[pcbeard]

I used vcpkg (installed at C:\dev\vcpkg) to build SDL last night, and it does seem like a reasonable way to automate these builds. What I don't quite understand yet is whether it can provide equivalent pkg-config functionality. I did notice that you can install a version of dpkgconfig using vcpkg.

Looking at your build failure, it seems that vcpkg doesn't actually put built products into the path like homebrew does. You have to sort of know where the built-products go, which is a pain.

Trying to debug why swift build is failing on windows, I looked at swift build --verbose on Debian. As part of its output, it prints out the line:

pkg-config --variable pc_path pkg-config

If I run that on Debian, it prints out the default path that is being used to find .pc files:

pkg-config --variable pc_path pkg-config
/home/linuxbrew/.linuxbrew/lib/pkgconfig:/home/linuxbrew/.linuxbrew/share/pkgconfig:/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/os/linux/pkgconfig

Running it on Windows:

 pkgconf --variable pc_path pkg-config
../lib/pkgconfig;../share/pkgconfig

I wonder if Swift build is trying to do something with these paths, and failing on Windows. The result of pkgconf with sdl2 looks fine however:

 pkgconf --cflags sdl2
-IC:/dev/vcpkg/installed/x64-windows/include/SDL2

As a further experiment, if you look in my branch vcpkg-sdl, you'll see I'm hard coding the paths for the Windows headers as installed by vcpkg. With these steps I am able to get CSDL2 to build. However, if I try to run the tests, they don't run. Presumably because they aren't finding the .dll at runtime.

Confirmed, if I hand copy /dev/vcpkg/installed/x64-windows/bin/SDL2.dll into the debug directory, then the tests run.

[pcbeard]

Since SPM support for pkg-config on Windows seems to be hosed (using local git repos also seems weirdly broken too), I move that we write a script to generate the shim.h file, using the output of pkg-config --variable includedir. We'll also want to copy the .lib and .dll files into the .build/<configuration> directory.

[ctreffs]

PR adds windows CI without really resting this package.
#9 will provide the implementation for that.