curl_multi_setopt(3) on macOS

The text fe7f0164b9f4, as OS X 10.11.6, OS X 10.11.5, OS X 10.11.4, OS X 10.11.3, OS X 10.11.2, OS X 10.11.1, OS X 10.11, OS X 10.10.5 shipped it.

set options for a curl multi handle

curl_multi_setopt(3) on macOS now