add nix flake for development shell

It does not support cross compilation but requires x86_64-linux.

.envrc

@@ -0,0 +1 @@
+use flake path:./nix

.gitignore

@@ -0,0 +1,3 @@
+/obj
+/files/cfg
+/.direnv

nix.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env sh
+
+# Run a shell inside of the nix-based development environment.
+
+set -eu
+
+cd "$(dirname "$0")"
+
+exec nix --experimental-features 'nix-command flakes' develop path:./nix --command "${SHELL:-sh}"

nix/flake.lock

@@ -0,0 +1,25 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1747467164,
+        "narHash": "sha256-JBXbjJ0t6T6BbVc9iPVquQI9XSXCGQJD8c8SgnUquus=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "3fcbdcfc707e0aa42c541b7743e05820472bdaec",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

nix/flake.nix

@@ -0,0 +1,35 @@
+{
+  outputs =
+    { self, nixpkgs }:
+    let
+      shell =
+        { pkgs }:
+        pkgs.mkShell {
+          packages = with pkgs; [
+            ncurses
+            dialog
+            perl
+            bashInteractive
+            flex
+            bison
+            buildPackages.qemu
+          ];
+          shellHook = ''
+            # Do not attempt cross compilation.
+            export CROSS_COMPILE=
+            # These are set by the compiler’s shellHook but mk refuses to work when these are set.
+            unset CC CPP CXX AR AS LD
+          '';
+        };
+
+      lib = import "${nixpkgs}/lib";
+      forAll = list: f: lib.genAttrs list f;
+    in
+    {
+      devShells = forAll [ "x86_64-linux" ] (system: {
+        default = shell {
+          pkgs = nixpkgs.legacyPackages.${system};
+        };
+      });
+    };
+}