CURLOPT_HEADERDATA(3) on macOS

The text 0eb2a672ceeb, 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.

pointer to pass to header callback

CURLOPT_HEADERDATA(3) on macOS now