Add some extra makefile targets

The opusfile and install-opusfile targets let us build and install just
the library (without the docs, even if doxygen is present and enabled).

The docs and install-docs targets let us build and install just the docs
without the library.

The default for make and make install remains to build both if doxgen
is present and enabled.

Signed-off-by: Ralph Giles <giles@mozilla.com>