Use git revision in dist tarball name, if possible.

We still need a static version so builds from release
tarballs work, or checkouts where git itself is unavailable.
Don't remove distdir twice.
Add 'make distcheck' target.
Fix typo in error message.
Add a util for manipulating opus headers.

Currently it just dumps the contents of the identification header.
There's a stub for updating the output gain.
Implement rogg_opus -g to adjust the output gain.