This patch introduces a new build tag `nolibopusfile` that conditionally
compiles out the code that imports `libopusfile`. This enables a static
binary build on Alpine Linux, which doesn’t have a static `libopusfile`.
Tests still work:
```sh
go test -tags nolibopusfile ./...
go test ./...
```
We could also have moved all libopusfile related code (i.e. Stream) into
a separate sub-package, but that would break backwards compatibility.
This feature is too unpopular to justify introducing a new major
version.
See also: https://github.com/hraban/opus/pull/24
pkg-config --cflags always puts you in the opus/ directory where opus.h
directly lives. No need to prefix with opus/. That just happened to work
because it was symlinked into /usr/include/opus, but we don't need to
rely on that now that we're using pkg-config.
Based on Tobias Wellnitz's comment (hraban/opus#12)
