Check for getaddrinfo in the winsock headers.

This makes us link directly to getaddrinfo if it is available, making
binaries incompatible with anything prior to windows XP. If compatibility
is desired, compile with --target-os=mingw32-prexp.

Patch by Martin Storsjö <martin at martin dot st>.

Originally committed as revision 21442 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2533,6 +2533,8 @@ if enabled network; then
             network_extralibs="-lws2" || \
         { check_func_headers winsock2.h closesocket -lws2_32 && \
             network_extralibs="-lws2_32"; }
+        # getaddrinfo is available if _WIN32_WINNT >= 0x0501 is defined
+        check_func_headers ws2tcpip.h getaddrinfo $network_extralibs
         check_type ws2tcpip.h socklen_t
         check_type ws2tcpip.h "struct addrinfo"
         check_type ws2tcpip.h "struct ipv6_mreq"