Make the default fonts conditional on services.xserver.enable

We were pulling in 44 MiB of fonts in the default configuration, which
is a bit excessive for headless configurations like EC2
instances. Note that dejavu_minimal ensures that remote X11-forwarded
applications still have a basic font regardless.

nixos/modules/config/fonts/fonts.nix

@@ -11,17 +11,27 @@ with lib;
       # TODO: find another name for it.
       fonts = mkOption {
         type = types.listOf types.path;
+        default = [];
         example = literalExample "[ pkgs.dejavu_fonts ]";
         description = "List of primary font paths.";
       };
 
+      enableDefaultFonts = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Enable a basic set of fonts providing several font styles
+          and families and reasonable coverage of Unicode.
+        '';
+      };
+
     };
 
   };
 
   config = {
 
-    fonts.fonts =
+    fonts.fonts = mkIf config.fonts.enableDefaultFonts
       [
         pkgs.xorg.fontbhlucidatypewriter100dpi
         pkgs.xorg.fontbhlucidatypewriter75dpi

nixos/modules/services/x11/xserver.nix

@@ -654,6 +654,8 @@ in
         ${xrandrMonitorSections}
       '';
 
+    fonts.enableDefaultFonts = mkDefault true;
+
   };
 
 }