darwinssl: fix build error in crypto authentication under Snow Leopard

It turns out Snow Leopard not only has SecItemCopyMatching() defined in
a header not included by the omnibus header, but it won't work for our
purposes, because searching for SecIdentityRef objects wasn't added
to that API until Lion. So we now use the old SecKeychainSearch API
instead if the user is building under, or running under, Snow Leopard.

Bug: http://sourceforge.net/p/curl/bugs/1255/
Reported by: Edward Rudd