lavf: default .ogg audio to libvorbis if we can

Since 2007, the Xiph.org Foundation recommends that .ogg only be used
for Ogg Vorbis audio files.

Source: http://wiki.xiph.org/index.php/MIME_Types_and_File_Extensions

However we only do it if we have libvorbis available because the
built in vorbis encoder is not as good.
Signed-off-by: NVittorio Giovara <vittorio.giovara@gmail.com>

libavformat/oggenc.c

@@ -616,7 +616,8 @@ AVOutputFormat ff_ogg_muxer = {
     .mime_type         = "application/ogg",
     .extensions        = "ogg,ogv,spx,opus",
     .priv_data_size    = sizeof(OGGContext),
-    .audio_codec       = AV_CODEC_ID_FLAC,
+    .audio_codec       = CONFIG_LIBVORBIS_ENCODER ?
+                         AV_CODEC_ID_VORBIS : AV_CODEC_ID_FLAC,
     .video_codec       = AV_CODEC_ID_THEORA,
     .write_header      = ogg_write_header,
     .write_packet      = ogg_write_packet,