Add spec aarch64-apple-darwin9 (Apple Silicon)

[iamFIREcracker]

This PR adds specs for Apple Silicon, finally making it possible to run Sketch on newer macs (see related links below). Also, tested on a MacBook Air M2 and it all seems to be working just fine.

Steps to regenerate the specs (I used nix)

First off, force quicklisp to use a version of cl-sdl2 which supports Apple Silicon (e.g., you can clone my fork into ~/quicklisp/local-projects, or wherever your quicklisp installation lives). Note: you won't need this step anymore once changes are merged upstream (see: lispgames/cl-sdl2#167).

Next, drop the following lines in a file named shell.nix at the root of the repository, and then, in the same directory, run nix-shell (this will take care of installing SDL2_image and c2ffi).

shell.nix

let
  pkgs =
    import (
      fetchTarball {
        name = "nixos-23.05_2023-09-18";
        url =
          "https://github.com/NixOS/nixpkgs/archive/f81112bbb0f357b80333300c4702ff7538096156.tar.gz";
        sha256 = "0pxhiggxl5drnpva9287w136rrykcd7xk1nyh655wgf6sb38a82k";
      }
    )
    {};
in
  pkgs.mkShell {
    packages = [
      pkgs.SDL2
      pkgs.SDL2_image
      pkgs.c2ffi
    ];

    LD_LIBRARY_PATH = "${pkgs.SDL2}/lib:${pkgs.SDL2_image}/lib";
    /* SDL2/SDL2_image would import SDL like "SDL.h" and not like "SDL2/SDL.h"
       so to let c2ffi resolve that import correctly, we need to include
       ${pkgs.SDL2.dev}/include/SDL2 and not ${pkgs.SDL2.dev}/include */
    C_INCLUDE_PATH = "${pkgs.SDL2.dev}/include/SDL2:${pkgs.SDL2_image}/include";
  }

Remove the already existing specs (autowrap will try to generate the spec files only if missing):

$ rm -f src/spec/SDL_image.*.spec

Load :SDL2-IMAGE (this will get autowrap to generate the various spec files:

$ sbcl \
	--load "sdl2-image.asd" \
	--eval "(ql:quickload '(:sdl2-image))" \
	--eval "(uiop:quit)"

PS. I also changed autowrap to allow parsing of C_INCLUDE_PATH env variable; let me know if you preferred that to be moved into a separate PR instead.

[fitzsim]

Hmm, OK, but can you make the inclusion of :sysincludes conditional on C_INCLUDE_PATH being set? I do not know what :sysincludes nil does to other platforms. Also, can you document how to use this, even just in a comment? Does nix shell automatically export C_INCLUDE_PATH or is the idea the user has to set it prior to doing something?

[iamFIREcracker]

I could not find a way to set :SYSINCLUDES conditionally to the presence of the C_INCLUDE_PATH environment variable; however, I did take a look at the C-INCLUDE macro-expansion, and sysincludes just gets EVAL-d, and since it's initialized with NIL, it should not make a difference if we pass an empty list or omit it altogether.

Also, to address your question about C_INCLUDE_PATH, I went on and added a new section to the README explaining how to regenerate bindings and how C_INCLUDE_PATH can be used if the default configuration was not working.