vgmstream: Install the Audacious plugin to a separate output

Instead of building vgmstream separately with and without the Audacious
plugin, build it only once and install the binaries and the Audacious
plugin in separate outputs.

Also, this way, audacious-plugins no longer has the vgmstream binaries
in its closure.

pkgs/by-name/au/audacious-plugins/package.nix

@@ -110,9 +110,7 @@ stdenv.mkDerivation rec {
   dontWrapQtApps = true;
 
   postInstall = ''
-    ln -s ${
-      vgmstream.override { buildAudaciousPlugin = true; }
-    }/lib/audacious/Input/* $out/lib/audacious/Input
+    ln -s ${vgmstream.audacious}/lib/audacious/Input/* $out/lib/audacious/Input
   '';
 
   meta = audacious-bare.meta // {

pkgs/by-name/vg/vgmstream/package.nix

@@ -12,7 +12,6 @@
   libao,
   speex,
   nix-update-script,
-  buildAudaciousPlugin ? false, # only build cli by default, pkgs.audacious-plugins sets this to enable plugin support
 }:
 
 stdenv.mkDerivation rec {
@@ -34,10 +33,16 @@ stdenv.mkDerivation rec {
     ];
   };
 
+  outputs = [
+    "out"
+    "audacious"
+  ];
+
   nativeBuildInputs = [
     cmake
     pkg-config
-  ] ++ lib.optional buildAudaciousPlugin gtk3;
+    gtk3
+  ];
 
   buildInputs = [
     mpg123
@@ -45,17 +50,18 @@ stdenv.mkDerivation rec {
     libvorbis
     libao
     speex
-  ] ++ lib.optional buildAudaciousPlugin audacious-bare;
+    audacious-bare
+  ];
 
   preConfigure = ''
     substituteInPlace cmake/dependencies/audacious.cmake \
-      --replace "pkg_get_variable(AUDACIOUS_PLUGIN_DIR audacious plugin_dir)" "set(AUDACIOUS_PLUGIN_DIR \"$out/lib/audacious\")"
+      --replace "pkg_get_variable(AUDACIOUS_PLUGIN_DIR audacious plugin_dir)" "set(AUDACIOUS_PLUGIN_DIR \"$audacious/lib/audacious\")"
   '';
 
   cmakeFlags = [
     # It always tries to download it, no option to use the system one
     "-DUSE_CELT=OFF"
-  ] ++ lib.optional (!buildAudaciousPlugin) "-DBUILD_AUDACIOUS=OFF";
+  ];
 
   meta = with lib; {
     description = "Library for playback of various streamed audio formats used in video games";