Build gloox with GnuTLS on macOS, refs #4705.
This includes GMP and nettle, that are dependencies of GnuTLS.
On versions of OSX/macOS up to 10.11, TLS handshakes can still fail and crash, so users of those older versions should disable TLS on the lobby in the options screen, and will still be able to use it.
Differential Revision: https://code.wildfiregames.com/D1654
Tested By: Tobbi, trompetin17, and testers of their bundles.