overlays(firebird-emu): add overlay for darwin support

overlays/default/firebird-emu.nix

@@ -0,0 +1 @@
+import ./firebird-emu/default.nix

overlays/default/firebird-emu/default.nix

@@ -0,0 +1,3 @@
+self: super: {
+  firebird-emu = self.libsForQt5.callPackage ./package.nix {};
+}

overlays/default/firebird-emu/package.nix

@@ -0,0 +1,44 @@
+{
+  stdenv,
+  lib,
+  fetchFromGitHub,
+  qmake,
+  qtbase,
+  qtdeclarative,
+  qtquickcontrols,
+  wrapQtAppsHook,
+}:
+stdenv.mkDerivation rec {
+  pname = "firebird-emu";
+  version = "1.6";
+
+  src = fetchFromGitHub {
+    owner = "nspire-emus";
+    repo = "firebird";
+    rev = "v${version}";
+    fetchSubmodules = true;
+    hash = "sha256-ZptjlnOiF+hKuKYvBFJL95H5YQuR99d4biOco/MVEmE=";
+  };
+
+  NIX_CFLAGS_COMPILE = lib.optionals (stdenv.cc.isClang && stdenv.isDarwin) [
+    "-fno-lto" # work around https://github.com/NixOS/nixpkgs/issues/19098
+  ];
+
+  nativeBuildInputs = [qmake wrapQtAppsHook];
+
+  buildInputs = [qtbase qtdeclarative qtquickcontrols];
+
+  postInstall = lib.optionalString stdenv.hostPlatform.isDarwin ''
+    mkdir $out/Applications
+    mv $out/bin/${pname}.app $out/Applications/
+  '';
+
+  meta = {
+    homepage = "https://github.com/nspire-emus/firebird";
+    description = "Third-party multi-platform emulator of the ARM-based TI-Nspire™ calculators";
+    license = lib.licenses.gpl3;
+    maintainers = with lib.maintainers; [pneumaticat];
+    # Only tested on Linux, but likely possible to build on, e.g. macOS
+    platforms = lib.platforms.unix;
+  };
+}