only try to use the bundled vorbis libs if BUNDLED_OGG_VORBIS is set

Only try to use the bundled vorbis libs if BUNDLED_OGG_VORBIS is set, otherwise the bundled libs will be used when FEATURE_OGG_VORBIS is set.  And as FEATURE_OGG_VORBIS is on by default currently cmake will always look for the bundled libraries.