system76-keyboard-configurator: build on darwin

The build produces a binary that is usable. The build does not produce
a native .app bundle because the build system depends on iconutil from
Xcode, which is impure.

Even if we switch to icnsutil, there are more issues with their
`deploy.py` script that would require a complete rewrite to accommodate
nix.

pkgs/by-name/sy/system76-keyboard-configurator/package.nix

@@ -1,5 +1,6 @@
 {
   lib,
+  stdenv,
   fetchFromGitHub,
   rustPlatform,
   gtk3,
@@ -36,12 +37,14 @@ rustPlatform.buildRustPackage rec {
     gtk3
     hidapi
     libusb1
+  ]
+  ++ lib.optionals stdenv.hostPlatform.isLinux [
     udev
   ];
 
   cargoHash = "sha256-0UmEWQz+8fKx8Z1slVuVZeiWN9JKjEKINgXzZ6a4jkE=";
 
-  postInstall = ''
+  postInstall = lib.optionalString stdenv.hostPlatform.isLinux ''
     install -Dm444 linux/com.system76.keyboardconfigurator.desktop -t $out/share/applications
     cp -r data/icons $out/share
   '';
@@ -52,6 +55,6 @@ rustPlatform.buildRustPackage rec {
     homepage = "https://github.com/pop-os/keyboard-configurator";
     license = with lib.licenses; [ gpl3Only ];
     maintainers = with lib.maintainers; [ mirrexagon ];
-    platforms = lib.platforms.linux;
+    platforms = with lib.platforms; linux ++ darwin;
   };
 }