Compiling against the 10.13 SDK with --with-macosx-version-min-
required set to 10.9 or 10.10 (or defaulted, meaning 10.9) causes a
compilation error now. Earlier SDKs did not catch it if you were using
connectx() even if targeting pre-10.11.

Try the approach from https://github.com/curl/curl/pull/1336/commits .

Change-Id: I7cac294931c8afa6ff26a6ca9cf4491aff249de0