- use AV_CODEC_ID_MPEG4 instead of CODEC_ID_MPEG4
* fix compilation issues with libav:
- use AV_CODEC_ID_MPEG4 instead of CODEC_ID_MPEG4
- added missing #include <libavutil/opt.h>
- fixed with "LIBAVFORMAT_VERSION_MICRO >= 100":
  libav does not support avformat_alloc_output_context2. 
  libav does not support avcodec_get_name.
* bugfix; coredump if no codec name was specified in url.