Motivation for this change

Adding Darwin support for gringo will allow aspcud to build for Darwin as well. This is important beause aspcud is the recommended dependency solver for OPAM, the OCaml package manager, and currently opam specifically avoids using aspcud on Darwin (as per this discussion).

If this PR is merged, I will follow up with PRs for aspcud and opam to get everything working on Darwin.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.