Add check for presence of XiphQT in exporter.

Prevents creation of empty audio track when the Xiph Vorbis encoder
component is not installed.

BUG=http://code.google.com/p/webm/issues/detail?id=386
TEST=Exporting from iMovie without a Vorbis encoder does not
     produce a WebM file with an audio track that contains no
     audio samples.

Change-Id: I7a9f1b161c96dc229e2284aa8c8640ff75eb72fa