libopus: Allow to enable custom modes.
libopus: Allow to enable custom modes.

The reason I'm not making this the default is because it seems to add
complexity and degrades performance of the library. For details have a
look at this lengthy discussion at:

https://bugs.debian.org/686777

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@redmoonstudios.org>

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