Add GAME option to Cmake builds

Allows for the removal of some hardcoded filenames, such as `flare.desktop`.
It also lets us set the `engine_folder` variable in Settings.cpp.

`flare` is the default for this variable, so flare-game can be built as
usual. However, here's how Polymorphable would be built:

```
cmake . -DGAME="polymorphable" -DDATA_DIR="share/games/polymorphable"
```