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.

demux/demux_lavf.c

@@ -560,9 +560,13 @@ static int demux_open_lavf(demuxer_t *demuxer, enum demux_check check)
         avfc->flags |= AVFMT_FLAG_GENPTS;
     if (opts->index_mode == 0)
         avfc->flags |= AVFMT_FLAG_IGNIDX;
+
+#if LIBAVFORMAT_VERSION_MICRO >= 100
     /* Keep side data as side data instead of mashing it into the packet
-     * stream. */
+     * stream.
+     * Note: Libav doesn't have this horrible insanity. */
     av_opt_set(avfc, "fflags", "+keepside", 0);
+#endif
 
     if (lavfdopts->probesize) {
         if (av_opt_set_int(avfc, "probesize", lavfdopts->probesize, 0) < 0)