ffms: symlink vapoursynth plugin

pkgs/development/libraries/ffms/default.nix

@@ -16,6 +16,13 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ pkgconfig ];
   buildInputs = [ zlib ffmpeg ];
 
+  # ffms includes a built-in vapoursynth plugin, see:
+  # https://github.com/FFMS/ffms2#avisynth-and-vapoursynth-plugin
+  postInstall = ''
+    mkdir $out/lib/vapoursynth
+    ln -s $out/lib/libffms2.so $out/lib/vapoursynth/libffms2.so
+  '';
+
   meta = with stdenv.lib; {
     homepage = https://github.com/FFMS/ffms2/;
     description = "Libav/ffmpeg based source library for easy frame accurate access";