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/.travis.yml

@@ -0,0 +1,38 @@
+language: c
+
+compiler:
+  - gcc
+  - clang
+
+os:
+  - linux
+  - osx
+
+osx_image: xcode11.3
+
+addons:
+  apt:
+    packages:
+      - doxygen
+      - libogg-dev
+      - libopus-dev
+      - libssl-dev
+  homebrew:
+    brewfile: true
+
+env: PKG_CONFIG_PATH=$PKG_CONFIG_PATH:/usr/local/opt/openssl/lib/pkgconfig
+
+before_script:
+  - if [ "$TRAVIS_OS_NAME" = "linux" ]; then
+      curl -O https://cmake.org/files/v3.16/cmake-3.16.9-Linux-x86_64.tar.gz;
+      tar xzf cmake-3.16.9-Linux-x86_64.tar.gz;
+      export PATH="${PWD}/cmake-3.16.9-Linux-x86_64/bin:${PATH}";
+    fi
+
+script:
+  - cmake -Bbuild -H.
+  - cmake --build build
+  - ./autogen.sh
+  - ./configure
+  - make
+  - make distcheck