Update macOS GitHub runner, SDL and extend compatibility (#217)

* Update SDL and SDL_mixer on macOS

* Extend (Intel) Mac compatibility all the way to OS X El Capitan

* Update macOS runner due to GitHub deprecation but still use and Xcode version that supports 10.11

CMakeLists.txt

@@ -23,6 +23,7 @@ if(APPLE)
     set(MACOSX_RPATH)
     set(CMAKE_BUILD_WITH_INSTALL_RPATH true)
     set(CMAKE_INSTALL_RPATH "@executable_path/../Frameworks")
+    set(CMAKE_OSX_DEPLOYMENT_TARGET "10.11")
     set(CMAKE_OSX_ARCHITECTURES "arm64;x86_64")
     list(APPEND SDL2_PATH "${CMAKE_CURRENT_LIST_DIR}/Libs")
     list(APPEND SDL2_MIXER_PATH "${CMAKE_CURRENT_LIST_DIR}/Libs")