demux_lavf: work around idiotic libavutil API printing a warning

Trying to set a non-existent flag (like +keepside on Libav) causes
libavutil print an incomprehensible warning (something about eval;
probably the overengineered libavutil option parser tripping over the
'+' normally used for flags, and trying to interpret it as formula).

There's apparently no easy way to check for the existence of a flag,
so add some more ifdeffery to shut it up.