This subdir compiles on its own, but it needs the mixer include files in
../mixer/ ATM. I first hacked mixer/CMakeLists.txt to install the mixer
headers but it's cleaner the way it is now.

It compiles, and that's all the testing it has seen.
Too many other things (like kdelibs) were broken, and I don't know how to get
a real device either.
The API is really easy to use together with Qt's, as the method names are quite
similar. Thumbs up!