add a flake.nix file for the development environment, add direnv-related files to .gitignore

flake.nix

@@ -0,0 +1,70 @@
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = {
+    self,
+    nixpkgs,
+    flake-utils,
+  }:
+    flake-utils.lib.eachDefaultSystem (system: let
+      pkgs = nixpkgs.legacyPackages.${system};
+      libselinuxPath = with pkgs;
+        lib.makeLibraryPath [
+          libselinux
+        ];
+      libaclPath = with pkgs;
+        lib.makeLibraryPath [
+          acl
+        ];
+
+      build_deps = with pkgs; [
+          clang
+          llvmPackages.bintools
+          rustup
+
+          pre-commit
+
+          # debugging
+          gdb
+        ];
+      gnu_testing_deps = with pkgs; [
+          autoconf
+          automake
+          bison
+          gnum4
+          gperf
+          gettext
+          texinfo
+        ];
+    in {
+      devShell = pkgs.mkShell {
+        buildInputs = build_deps ++ gnu_testing_deps;
+
+        RUSTC_VERSION = "1.75";
+        LIBCLANG_PATH = pkgs.lib.makeLibraryPath [pkgs.llvmPackages_latest.libclang.lib];
+        shellHook = ''
+          export PATH=$PATH:''${CARGO_HOME:-~/.cargo}/bin
+          export PATH=$PATH:''${RUSTUP_HOME:-~/.rustup}/toolchains/$RUSTC_VERSION-x86_64-unknown-linux-gnu/bin/
+        '';
+
+        SELINUX_INCLUDE_DIR = ''${pkgs.libselinux.dev}/include'';
+        SELINUX_LIB_DIR = libselinuxPath;
+        SELINUX_STATIC = "0";
+
+        # Necessary to build GNU.
+        LDFLAGS = ''-L ${libselinuxPath} -L ${libaclPath}'';
+
+        # Add precompiled library to rustc search path
+        RUSTFLAGS =
+          (builtins.map (a: ''-L ${a}/lib'') [
+            ])
+          ++ [
+            ''-L ${libselinuxPath}''
+            ''-L ${libaclPath}''
+          ];
+      };
+    });
+}