Problem: no way to specify source interface for UDP multicast sender …

[simias]

…sockets

Solution: if a binding interface is provided in the sender URL we pass
it to IP[V6]_MULTICAST_IF setsockopt

[bluca]

Windows build seems unhappy:

C:\projects\libzmq\src\udp_engine.cpp(164): error C2664: 'int setsockopt(SOCKET,int,int,const char *,int)' : cannot convert argument 4 from 'in_addr *' to 'const char *' [C:\projects\build_libzmq\libzmq.vcxproj]
          Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast

[simias]

I always forget about that