bs1770gain: init at 0.4.7

I've been trying to package this a while ago but found out that apart
from the hand-rolled build system a few sources were missing I didn't
continue to work on it.

This was the WIP expression:

https://gist.github.com/aszlig/c271f294410cc5af0f0a

Now, since the latest upstream version, the project uses autoconf and
automake, which makes it very much painless for us.

So now I can watch demoscene videos without getting hearing loss because
of volume differences :-)

Also, this might be useful for beets as well, in case they're going to
move away from using python-audiotools:

https://github.com/sampsyo/beets/issues/1342#issuecomment-86807500



Signed-off-by: aszlig <aszlig@redmoonstudios.org>