set SDLMIXER_VENDORED to ON and vendor the required libraries into libs/SDL3_mixer/external

disable SDLMIXER_VORBIS_VORBISFILE
add ogg v1.3.5-SDL
add opus v1.4.x-SDL
add opusfile v0.13-git-SDL

libs/SDL3_mixer/external/opus/autogen.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+# Copyright (c) 2010-2015 Xiph.Org Foundation and contributors.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the COPYING file.
+
+# Run this to set up the build system: configure, makefiles, etc.
+set -e
+
+srcdir=`dirname $0`
+test -n "$srcdir" && cd "$srcdir"
+
+echo "Updating build configuration files, please wait...."
+
+autoreconf -isf