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 */