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/opusfile/ci/unix.sh

@@ -0,0 +1,23 @@
+# Continuous integration build script for opusfile.
+# This script is run by automated frameworks to verify commits
+# see https://mf4.xiph.org/jenkins/job/opusfile-unix/
+
+# This is intended to be run from the top-level source directory.
+
+set -x
+
+# WARNING: clobbers outside the current tree!
+rm  -f ../opus
+ln -s /srv/jenkins/jobs/opus/workspace ../opus
+
+# compile
+make -C unix PKG_CONFIG_PATH=$PWD/../opus clean
+make -C unix PKG_CONFIG_PATH=$PWD/../opus
+
+# run any built-in tests
+make -C unix PKG_CONFIG_PATH=$PWD/../opus check
+
+# build the documentation
+make -C doc
+# currently fails on jenkins (debian stretch)
+# make -C doc/latex