cmake: Build shared libraries

cmake: Build shared libraries

The days of static builds are finally over. Build dynamic libraries
for all Quassel modules.

Let CMake handle the RPATH in a way that doesn't cause inconvenience;
always set an RPATH in the build tree so the correct libraries
are loaded, and also set an RPATH for installed binaries unless
they go into a system location.

Also define output directories for all build artifacts.