This diff ovverides the default flags for 2 of the built-in
configurations for CMake:
- The first one (`RelWithDebInfo`) is the default one, and is
equivalent to the `configure` default. It adds the `-g -O2` optimization
flags to the C and CXX compilers.
- The second one is the `Debug` configuration, which can be selected
by passing `-DCMAKE_BUILT_TYPE=Debug` to CMake. It is equivalent to the
`--enable-debug` flag for the `configure` script. It adds the `-g3 -Og`
(or fallback equivalents) flags to the CXX compiler, and defines `DEBUG`
and `DEBUG_LOCKORDER`.