macOS: Fix missing connectx function with Xcode version older than 9.0

The previous fix https://github.com/curl/curl/pull/1788 worked just for
Xcode 9. This commit extends the fix to older Xcode versions effectively
by not using connectx function.

Fixes https://github.com/curl/curl/issues/1330
Fixes https://github.com/curl/curl/issues/2080
Closes https://github.com/curl/curl/pull/1336
Closes #2082