Fix detection of IPv6 support on Darwin by ensuring that

IN6_IS_ADDR_MULTICAST gets an argument of type (const struct in6_addr *).
patch by Dan Villiom Podlaski Christiansen, danchr.at.daimi.dot.au.dot.dk

Originally committed as revision 4943 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1234,7 +1234,7 @@ int main( void ) {
   struct ipv6_mreq mreq6;
   getaddrinfo(0,0,0,0);
   getnameinfo(0,0,0,0,0,0,0);
-  IN6_IS_ADDR_MULTICAST(0);
+  IN6_IS_ADDR_MULTICAST((const struct in6_addr *)0);
 }
 EOF