CURLOPT_HTTPHEADER(3) on macOS

The text 8b9f78f8a5dd, 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 custom HTTP headers

CURLOPT_HTTPHEADER(3) on macOS now