For some reason, Mac OS X has a <netinet.h> that doesn't define
IPV6_PKTINFO unless one of __APPLE_USE_RFC_2292 or __APPLE_USE_RFC_3542
is set.  There is no default.  We should #define __APPLE_USE_RFC_3542
before including <netinet.h> if we want to get IPV6_PKTINFO on Mac OS X.

This is with Xcode 7.2 on Mac OS 10.10.5.