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: Vittorio Giovara <vittorio.giovara@gmail.com>