Fix FTBFS on OS X >=10.7 Need to select a IPv6 sockopt API.

src/config.h

@@ -254,7 +254,10 @@ HAVE_SOCKADDR_SA_LEN
 #define HAVE_SOCKADDR_SA_LEN
 /* Define before sys/socket.h is included so we get socklen_t */
 #define _BSD_SOCKLEN_T_
- 
+/* Select the RFC_3542 version of the IPv6 socket API. 
+   Define before netinet6/in6.h is included. */
+#define __APPLE_USE_RFC_3542 
+
 #elif defined(__NetBSD__)
 #define HAVE_BSD_NETWORK
 #define HAVE_GETOPT_LONG