on inter-library dependency magic
- libavcodec needs libvorbis which needs libogg, so add ogg
to libavcodec's dependency libs
- as of the previous commit, libavformat depends on libX11 and
libXext, add them as dependency libs
- add RPATH info when linking libavcodec and libavformat, so
applications that dlopen() libavcodec or libavformat have a
better chance of finding dependency libs
fixes multimedia/ffmpeg2theora build problem noticed by sturm@
7d42d3b257