Add flake.nix file for Nix/NixOS users

This flake defines a development shell for developing argbash. It
ensures that all dependencies used to build this project are pinned to a
specific version (via a lock file). In this case, make, m4, and autoconf.

This guarantees deterministic build behavior; builds always use the same
exact dependencies until the lock file is updated. Any updates to the
dependencies on a system level will not be able to break our build if an
API or a behavior of such a dependency has changed.

The nixpkgs repository already has a build script for argbash here:

https://github.com/NixOS/nixpkgs/tree/master/pkgs/by-name/ar/argbash

But this is just for building the tool so it can be used. A devshell can
be defined and used from there, but this allows it to be updated when
new dependencies come in.

Author: kstrafe

flake.lock

@@ -0,0 +1,25 @@
+{
+  nixConfig.bash-prompt-suffix = "\[nix\] ";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+
+  outputs = inputs:
+    inputs.flake-utils.lib.eachDefaultSystem (system:
+    let
+      pkgs = import inputs.nixpkgs { inherit system; };
+    in
+      {
+        devShells.default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            autoconf
+            gnumake
+            sphinx
+          ];
+        };
+      }
+    );
+}