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>.

source/cdns.c

@@ -21,9 +21,6 @@
  * $Id$
  */
 
-#include <sys/types.h>
-#include <netinet/in.h>
-
 #include "cdns.h"
 #include "irc.h"	/* To pick up our next #define checks */
 static char cvsrevision[] = "$Id$";