vapoursynth: fix darwin build

pkgs/development/libraries/vapoursynth/default.nix

···
 { stdenv, fetchFromGitHub, pkgconfig, autoreconfHook,
-  zimg, libass, yasm, python3,
+  zimg, libass, yasm, python3, libiconv, ApplicationServices,
   ocrSupport ?  false, tesseract,
   imwriSupport? true,  imagemagick7
 }:
···
     sha256 = "0nabl6949s7awy7rnr4ck52v50xr0hwr280fyzsqixgp8w369jn0";
   };
 
+  nativeBuildInputs = [ pkgconfig autoreconfHook ];
   buildInputs = [
-    pkgconfig autoreconfHook
     zimg libass tesseract yasm
     (python3.withPackages (ps: with ps; [ sphinx cython ]))
-  ] ++ optional ocrSupport   tesseract
+  ] ++ optionals stdenv.isDarwin [ libiconv ApplicationServices ]
+    ++ optional ocrSupport   tesseract
     ++ optional imwriSupport imagemagick7;
 
   configureFlags = [

pkgs/top-level/all-packages.nix

···
 
   vapor = callPackage ../games/vapor { love = love_0_8; };
 
-  vapoursynth = callPackage ../development/libraries/vapoursynth { };
+  vapoursynth = callPackage ../development/libraries/vapoursynth {
+    inherit (darwin.apple_sdk.frameworks) ApplicationServices;
+  };
 
   vapoursynth-mvtools = callPackage ../development/libraries/vapoursynth-mvtools { };