Use SHLIBOBJS and STLIBOBJS in the Makefiles for avcodec and avformat,
and add a stub ffjni.c to libavformat, which allows the symbols to be
duplicated for shared builds but not static builds.
Signed-off-by: Leo Izen <leo.izen@gmail.com>
Signed-off-by: Matthieu Bouron <matthieu.bouron@gmail.com>