#ifdef reduction infrastructure, based on an argument between Shaun Jackman,

Rob Landley, and others.

Currently CONFIG options are defined or undefined, so we chop out code with
#ifdefs, ala:
#ifdef CONFIG_THING
  stuff();
#endif

This creates a new header file, bb_config.h, which sets the CONFIG entry to 1
or 0, and lets us do:

  if(CONFIG_THING) stuff();

And let the compiler do dead code elimination to get rid of it.  (Note: #ifdef
will still work because for the 1 case it's a static const int, not a #define.)

networking/ifconfig.c

@@ -37,6 +37,7 @@
 #include <string.h>		/* strcmp and friends */
 #include <ctype.h>		/* isdigit and friends */
 #include <stddef.h>		/* offsetof */
+#include <unistd.h>
 #include <netdb.h>
 #include <sys/ioctl.h>
 #include <net/if.h>
@@ -558,6 +559,7 @@ int ifconfig_main(int argc, char **argv)
 		continue;
 	}					/* end of while-loop */
 
+	if (CONFIG_FEATURE_CLEAN_UP) close(sockfd);
 	return goterr;
 }