slirp: Avoid statements without effect on Big Endian host

Darwin has HTON*/NTOH* macros that on BE simply return the argument.
This is incompatible with SLIRP's use of these macros as a statement.

Undefine the macros in the HOST_WORDS_BIGENDIAN code path to redefine
these macros as no-op, as already done when they were undefined.
Suggested-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>

slirp/ip.h

@@ -34,18 +34,14 @@
 #define _IP_H_
 
 #ifdef HOST_WORDS_BIGENDIAN
-# ifndef NTOHL
-#  define NTOHL(d)
-# endif
-# ifndef NTOHS
-#  define NTOHS(d)
-# endif
-# ifndef HTONL
-#  define HTONL(d)
-# endif
-# ifndef HTONS
-#  define HTONS(d)
-# endif
+# undef NTOHL
+# undef NTOHS
+# undef HTONL
+# undef HTONS
+# define NTOHL(d)
+# define NTOHS(d)
+# define HTONL(d)
+# define HTONS(d)
 #else
 # ifndef NTOHL
 #  define NTOHL(d) ((d) = ntohl((d)))