Enable IPv6 for macOS

[tstenner]

In api_config.cpp, IPv6 is disabled on Macs by default:

		std::string ipv6_str = pt.get("ports.IPv6",
#ifdef __APPLE__
		"disable"); // on Mac OS (10.7) there's a bug in the IPv6 implementation that breaks LSL when it tries to use both v4 and v6

In the Git history, this file was changed twice: first, when @dmedine copied the source code to Github and then again when the formatting was changed automatically. So, I have no idea what has been the problem, if it still happens on more recent macOS versions or if we can remove the workaround.