Build fails because SDL2 is not found

Running CMake on the src directory produces this output (on Mac OS):

```
CMake Error at CMakeLists.txt:85 (find_package):
By not providing "FindSDL2.cmake" in CMAKE_MODULE_PATH this project has
asked CMake to find a package configuration file provided by "SDL2", but
CMake did not find one.

Could not find a package configuration file provided by "SDL2" with any of
the following names:

SDL2Config.cmake
sdl2-config.cmake
```

The instructions don't mention this; the only dependency mentioned is Triangle, which I've copied into the specified place.