Improve Kea compilation time
While playing around with kea compilation times, I found couple things that can be done to improve compilation time. This ticket is about
- config.h is breaking down ccache speed-up. It includes git commit-id, which is different with every build. That file is included everywhere, which causes ccache to have a very poor hit rate.
- we build by default with debug symbols. This slows down compilation by almost 3 minutes on my (@tomek) dev system