MaNGOS uses a 3rd party C++ Sockets library and MACOSX is defined in a Makefile.Defines file, which MaNGOS doesn't use. Your patch isn't complete because MACOSX is also tested in these additional files:
dep/include/sockets/UdpSocket.h
dep/include/sockets/UdpSocket.cpp
dep/src/sockets/SocketHandler.cpp
Since any changes made can be lost the next time MaNGOS updates the sockets library, a more simpler patch to a common file could be something like:
Index: dep/include/sockets/sockets-config.h
===================================================================
--- dep/include/sockets/sockets-config.h (revision 6216)
+++ dep/include/sockets/sockets-config.h (working copy)
@@ -38,6 +38,9 @@
#undef ENABLE_EXCEPTIONS
#endif // _RUN_DP
+#if defined(__APPLE__) && defined(__MACH__) && !defined(MACOSX)
+#define MACOSX
+#endif
/* OpenSSL support. */
//#define HAVE_OPENSSL
But from testing, it seems that further patches are needed to fix code that is now active (for Tiger atleast).