Provide packaging routines / CMake target

[nickelpro]

I was trying to add centurion to my vcpkg registry and was somewhat surprised that it didn't generate INSTALL targets.

So this patch is a first pass at packaging centurion in the "modern" tradition.

Notes:

• We generate a centurion.pc for anyone still attached to pkg-config
• We use file sets to ensure the headers are installed and linked correctly. This dodges a minor issue of integration as a subproject vs imported target, as file sets always generate the correct path information.
• Using file sets bumps the minimum required CMake version to 2.23
• We provide an alias for centurion::centurion so that subproject/imported target usage is the same
• Tests and examples use the new target, for demonstration purposes

Possible Future Work:

• Separate targets that don't require mixer/ttf/image
• Deprecate the set() vars? They're considered an anti-pattern in some CMake circles

A working portfile using this patch is here (commit).

If you want to test this out, the following vcpkg.json will do it:

{
  "$schema": "https://raw.githubusercontent.com/microsoft/vcpkg-tool/main/docs/vcpkg.schema.json",
  "name": "centurion-test",
  "version": "1.0.0",
  "dependencies": [
    "centurion"
  ],
  "vcpkg-configuration": {
    "default-registry": {
      "kind": "git",
      "baseline": "f6a5d4e8eb7476b8d7fc12a56dff300c1c986131",
      "repository": "https://github.com/microsoft/vcpkg.git"
    },
    "registries": [
      {
        "kind": "git",
        "baseline": "14ff6907873864ae70dd145097894c84da43a292",
        "repository": "https://github.com/nickelpro/vito-reg.git",
        "packages": [
          "centurion"
        ]
      }
    ]
  }
}

[nickelpro]

Second commit differentiates targets:

• centurion::centurion-headers-only: Bring your own SDL
• centurion::centurion: Use shared libraries if they exist, otherwise static
• centurion::centurion-static: Use static libraries, only available if static libraries exist

[nickelpro]

Third commit makes centurion::centurion and centurion::centurion-static optional, completely removing the find_package(REQUIRED) for SDL. It's possible to configure centurion as a pure headers lib and link in SDL manually from wherever

[nickelpro]

Fourth commit corrects a minor oversight about needing to link sdl2main

Fifth commit contains changes necessary for CI to work:

• Update Ubuntu hosts before installing dependencies, surprising you didn't run into this before. Upstream dev fails CI for me right now because of this
• Properly convert env paths to CMake native
• Drop Windows SDL versions which don't ship a config file. These are annoying but not impossible to test, but better to just test them on *Nix/MacOS. If it's highly desired to support this I'll add a hack.
• Pass the SDL2*DIR env args to find_package if available