gsettings-desktop-schemas: re-disable introspection if no emulator

e.g. when cross compiling from Linux to FreeBSD.

Fixes: 24723644ac ("gsettings-desktop-schemas: build gir when cross")

pkgs/development/libraries/gsettings-desktop-schemas/default.nix

@@ -3,6 +3,8 @@
 , pkg-config
 , glib
 , gobject-introspection
+, buildPackages
+, withIntrospection ? stdenv.hostPlatform.emulatorAvailable buildPackages
 , meson
 , ninja
 , python3
@@ -27,9 +29,14 @@ stdenv.mkDerivation rec {
     ninja
     pkg-config
     python3
+  ] ++ lib.optionals withIntrospection [
     gobject-introspection
   ];
 
+  mesonFlags = [
+    (lib.mesonBool "introspection" withIntrospection)
+  ];
+
   postPatch = ''
     chmod +x build-aux/meson/post-install.py
     patchShebangs build-aux/meson/post-install.py