CURLMOPT_PUSHDATA(3) on macOS

The text e1497100eac0, 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 push callback

CURLMOPT_PUSHDATA(3) on macOS now