CURLOPT_DEBUGDATA(3) on macOS

The text b4d57f5feee1, 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.

custom pointer for debug callback

CURLOPT_DEBUGDATA(3) on macOS now