Hmm, are you building "in source"? These files will only appear in your build folder. The recommended way to build using cmake is to do it "out of source":

```bash
git clone ...
cd Libevent
mkdir build
cd build
cmake ..
cmake --build .
```

Then the only untracked file you'll see is this `build/` directory. Getting a clean build tree then means you simply `rm -rf build` instead of having to create some elaborate clean script.