Problem: tcp_address.cpp won't compile on OS/X w/o ZMQ_HAVE_IFADDRS

This is rather insane since the code knows well enough what systems
support if_nametoindex. I blame this on over-use of autotools early
in libzmq's days.

Anyhow, this breaks gyp builds on OS/X.

Solution: add ZMQ_HAVE_IFADDRS to build/gyp/platform.hpp for OS/X.

builds/gyp/platform.hpp

@@ -43,6 +43,7 @@
 #elif defined ZMQ_HAVE_OSX
 #   define ZMQ_USE_KQUEUE 1
 #   define HAVE_POSIX_MEMALIGN 1
+#   define ZMQ_HAVE_IFADDRS 1
 #   define ZMQ_HAVE_SO_KEEPALIVE 1
 #   define ZMQ_HAVE_TCP_KEEPALIVE 1
 #   define ZMQ_HAVE_TCP_KEEPCNT 1