Add HAVE_*_H checks around inclusions of netinet/in.h and sys/socket.h

Only include <netinet/in.h> and <sys/socket.h> if the system supplies them.
This is necessary to try and get the mingw32 build working again.

No need to include <sys/types.h> and <netinet/in.h> from cdns.c - these are
already included by <irc.h>.

include/struct.h

@@ -19,7 +19,9 @@
 #include "hash.h"
 #include "config.h"
 #include "ssl.h"
+#ifdef HAVE_NETINET_IN_H
 #include <netinet/in.h>
+#endif
 
 /*
  * struct sockaddr_storage isn't avaiable on all ipv6-ready-systems, bleh ;(