Compiling and running Doomsday 3 with CMake

Doomsday 3 is built with CMake 3.1 (or later).

1. Clone the repository.
2. Run doomsday/build/scripts/build_deps.py build to download and compile the required dependencies.
3. Create an empty build directory.
4. Run CMake with the appropriate build options.
5. Build.

TBD