Implement standardized "use flags" for Nixpkgs

pkgs/top-level/options.nix

@@ -0,0 +1,128 @@
+{lib, stdenv}:
+
+with lib;
+
+# Define some user flags. Note that none of these should be mutually
+# exclusive. For instance enabling X11 should not imply that Wayland
+# is disabled.
+
+{
+
+  lang = mkOption {
+    default = "en";
+    type = types.string;
+    description = ''
+
+      The target language to compile packages with support for.
+
+    '';
+  };
+
+  withX11 = mkOption {
+    default = stdenv.isLinux; # also BSDs?
+    type = types.bool;
+    description = ''
+
+      Whether to compile packages with support for the X Window System.
+
+    '';
+  };
+
+  withWayland = mkOption {
+    default = stdenv.isLinux; # also BSDs?
+    type = types.bool;
+    description = ''
+
+      Whether to compile packages with support for the Wayland window system.
+
+    '';
+  };
+
+  withAlsa = mkOption {
+    default = stdenv.isLinux;
+    type = types.bool;
+    description = ''
+
+      Whether to compile packages with support for the ALSA sound system.
+
+    '';
+  };
+
+  withPulseAudio = mkOption {
+    default = stdenv.isLinux;
+    type = types.bool;
+    description = ''
+
+      Whether to compile packages with support for the PulseAudio sound system.
+
+    '';
+  };
+
+  withJack = mkOption {
+    default = false;
+    type = types.bool;
+    description = ''
+
+      Whether to compile packages with support for the JACK sound system
+
+    '';
+  };
+
+  withQt = mkOption {
+    default = true;
+    type = types.bool;
+    description = ''
+
+      Whether to compile packages with support for the Qt graphical toolkit
+
+    '';
+  };
+
+  withGTK = mkOption {
+    default = false;
+    type = types.bool;
+    description = ''
+
+      Whether to compile packages with support for the GTK graphical toolkit
+
+    '';
+  };
+
+  withApple = mkOption {
+    default = stdenv.isDarwin;
+    type = types.bool;
+    description = ''
+
+      Whether to compile packages with Apple's proprietary frameworks.
+
+      Note that this only supports macOS currently.
+
+    '';
+  };
+
+  withSystemd = mkOption {
+    default = stdenv.isLinux;
+    type = types.bool;
+    description = ''
+
+      Whether to compile packages with support for the Systemd init system.
+
+      Note that this specifically means compiling packages with the
+      /Systemd/ libraries. Unit files should always be included in
+      packages.
+
+    '';
+  };
+
+  allowUnfree = mkOption {
+    default = false;
+    type = types.bool;
+    description = ''
+
+      Whether to support unfree software. This includes everything not
+      considered free software by the Free Software Foundation.
+
+    '';
+  };
+
+}

pkgs/top-level/stage.nix

@@ -116,6 +116,17 @@ let
     lib.optionalAttrs allowCustomOverrides
       ((config.packageOverrides or (super: {})) super);
 
+  # Allow users to specify global use flags that are applied in all
+  # packages.
+  options = self: super:
+    let config' = (config // (if (config.headless or false)
+                              then { withGTK = false; withX11 = false;
+                                     withAlsa = false; withPulseAudio = false;
+                                     withJack = false; withWayland = false; }
+                              else {}));
+    in lib.mapAttrs (name: option: config'.${name} or option.default)
+              (import ./options.nix { inherit (self) lib stdenv; });
+
   # The complete chain of package set builders, applied from top to bottom.
   # stdenvOverlays must be last as it brings package forward from the
   # previous bootstrapping phases which have already been overlayed.
@@ -125,6 +136,7 @@ let
     stdenvAdapters
     trivialBuilders
     splice
+    options
     allPackages
     aliases
     configOverrides