CURLMOPT_TIMERFUNCTION(3) on macOS

The text 5f63a53be050, as macOS 10.12.5, macOS 10.12.4, macOS 10.12.3, macOS 10.12.2, macOS 10.12.1, macOS 10.12 shipped it.

set callback to receive timeout values

CURLMOPT_TIMERFUNCTION(3) on macOS now