curl_strequal(3) on macOS

The text c173b5fa7ed2, 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.

case insensitive string comparisons

curl_strequal(3) on macOS now