Build release library by default

This is way the old version did it, and the way DUNE does it (at least the official autotools version), so I figure that by the principle of least surprise it should be done like that here too.

This is a change that will have effect for all developers that haven't configured their IDE to explicitly pass -DCMAKE_BUILD_TYPE=Debug to the build. (For those who have, there should be no change).