MB-11202: Use SecureTransport for SSL on OS X

As libcurl has been updated to use the native SSL library (see
http://review.couchbase.org/#/c/37653/), update moxi (which also uses
libcurl) to link against the relevent library depending on platform.

Change-Id: I4109b55768b11333bfd3e122463caaab4f95ba86
Reviewed-on: http://review.couchbase.org/37721
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>