Use the configure check from r21351 and use it to properly define struct

Use the configure check from r21351 and use it to properly define struct
sockaddr (in case it's not missing) so it always works on the system that
we are defining it for, in a RFC-2553/3493-compliant way (i.e. containing
a ss_family field). which is used in udp.c. Patch by Martin Storsjö
<$firstname $firstname st>.

Originally committed as revision 21352 to svn://svn.ffmpeg.org/ffmpeg/trunk