This fixes the build to use the headers returned by pkg-config.
The opus library assumes the called will pass -I/path/to/opus
so #include <opus.h> will work.

It wasn't used, and version control has now preserved it forever,
 so no reason to keep it around.

It wasn't actually implemented.

Seeking and playback work, at least as far as the examples test
 them.
There are probably still lots of bugs.