With CMake, the game version is stored in a generated header file so
dependencies on it are kept track of automatically.
The game now compiles when using CMake but does not link.
Generated code dependencies are not hooked up yet and corresponding
targets need to be built manually.
Incidental changes along the way:
* Move `export_level.lua` to `tools/level_scripts/`
* Small reorganizations/fixes to existing `CMakeLists.txt` files
* Support passing absolute paths to level export
Some parts need to be cleaned up/removed/streamlined after the CMake
build can build the game on its own and compatibility with what the
old makefile does is no longer needed.
