Add FindLibevent.cmake
An example implementation in use by the open source project coturn
can be found here: https://github.com/coturn/coturn/blob/677e5dc5d31e2c2de98212cecbf372f0671f676b/cmake/FindLibevent.cmake
An example implementation in use by the open source project coturn
can be found here: https://github.com/coturn/coturn/blob/677e5dc5d31e2c2de98212cecbf372f0671f676b/cmake/FindLibevent.cmake