Use a special struct_ip.h to include <netinet/ip.h> and accoutrements.

The comment in struct_ip.h explains the reasoning for this. The AIX C library
uses #defines that change the names of members of struct ip, and conflict with
some existing code. (Notably struct ip_hdr in libdnet and IPv4Header::h in
libnetutil.) We can still use the AIX files if we include <netinet/ip.h> after
this other code has been preprocessed. That's hard to enforce when
<netinet/ip.h> is included from another header file; this new file allows
including it always late, and only where needed.

struct_ip.h

@@ -0,0 +1,45 @@
+/* The C library on AIX defines the names of various members of struct ip to
+   something else in <netinet/ip.h>:
+
+struct ip {
+	struct	ip_firstfour ip_ff;
+#define	ip_v	ip_ff.ip_fv
+#define	ip_hl	ip_ff.ip_fhl
+#define	ip_vhl	ip_ff.ip_fvhl
+#define	ip_tos	ip_ff.ip_ftos
+#define	ip_len	ip_ff.ip_flen
+
+   This breaks code that actually wants to use names like ip_v for its own
+   purposes, like struct ip_hdr in libdnet. The AIX definitions will work
+   if they are included late in the list of includes, before other code that
+   might want to use the above names has already been preprocessed. The
+   includes that end up defining struct ip are therefore limited to this
+   file, so it can be included in a .cc file after other .h have been
+   included. */
+
+/* Linux uses these defines in netinet/ip.h to use the correct struct ip */
+#ifndef __FAVOR_BSD
+#define __FAVOR_BSD
+#endif
+#ifndef __USE_BSD
+#define __USE_BSD
+#endif
+#ifndef _BSD_SOURCE
+#define _BSD_SOURCE
+#endif
+
+/* BSDI needs this to insure the correct struct ip */
+#undef _IP_VHL
+
+#ifndef NETINET_IN_SYSTM_H  /* This guarding is needed for at least some versions of OpenBSD */
+#include <netinet/in_systm.h> /* defines n_long needed for netinet/ip.h */
+#define NETINET_IN_SYSTM_H
+#endif
+#ifndef NETINET_IP_H  /* This guarding is needed for at least some versions of OpenBSD */
+#include <netinet/ip.h>
+#define NETINET_IP_H
+#endif
+
+#ifndef WIN32
+#include <netinet/ip_icmp.h>
+#endif