Use toolchain via cmake.

[Viech]

• Either create a CBSE.py switch that creates a helper cmake config that provides a variable with the files that need to be compiled (prefered as it's portable) and include that from within Unvanquished's cmake config or use wildcasts inside the latter to include all CBSE files automatically.
• Make Unvanquished's cmake run the code generator (and handle all relevant errors such as a non-initialized git submodule) so that you can clone and compile the game as usual.