Summary: We can't port "Debian netcat" to OS X without definining this.
Who knows, perhaps someday there will be a Debian GNU/Darwin.
Contributor: Loic Fosse <fosse@gostai.com>
Index: netcat-1.10/netcat.c
===================================================================
--- netcat-1.10.orig/netcat.c 2008-01-20 22:57:53.000000000 -0500
+++ netcat-1.10/netcat.c 2008-01-20 23:07:25.000000000 -0500
@@ -86,6 +86,10 @@
#include <unistd.h>
#endif
+#ifndef SOL_IP
+# define SOL_IP IPPROTO_IP
+#endif
+
/* handy stuff: */
#define SA struct sockaddr /* socket overgeneralization braindeath */
#define SAI struct sockaddr_in /* ... whoever came up with this model */