Thanks Martin. The MAC_OS_X_VERSION_10_5 macro ensures that Python is still compatible with Mac OS before version 10.5: if it's built on 10.4 or older, it locks around getaddrinfo (since it's not thread-safe there), and on 10.5 and later it doesn't lock.

I've built on a Mac OS 10.4 virtual machine and verified we'll still use the lock on that OS.

I think this is good enough, without a runtime check. We distribute prebuilt binaries for Mac OS 10.5+:

https://www.python.org/downloads/release/python-2711/

So those prebuilt Pythons should no longer lock around getaddrinfo. 

People who still use older Mac OSes (more than 12 years old!) will have to build Python themselves in order to get recent versions of Python, since we don't distribute binaries for them any more. Since they're building Python on old Mac OS, the MAC_OS_X_VERSION_10_5 macro will be undefined and they'll keep using the getaddrinfo lock.