OSX fixes

The OpenSSL version on OSX (at least on Mavericks) does not have the tls version method functions, so we need to detect OSX there. It doesn't look like there's a good way to put this into the compat file without adding yet another layer of indirection, so I put it directly in the ifdef.

The second commit is just to shut up the compiler. I already know Apple want me to use SecureTransport, I don't need it hiding any useful warnings or errors from my code.

The last one shouldn't be OSX-specific, but newer Go versions have gotten quite picky about C pointer types, so this makes them agree completely. I'm on 1.4 currently but I've seen similar issues on 1.3.