opus

cd doc && make is not the same as make -C doc

Fixes a regression in the recursive builds introduced in 5467ce7b3f8
that causes `make check-opus` to fail horribly.

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>
(cherry picked from commit be0d8c5ee11923688fdc620409c979dc13bfd697)