#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.)

include/libbb.h

@@ -41,7 +41,7 @@
 
 #include <features.h>
 
-#include "config.h"
+#include "bb_config.h"
 #ifdef CONFIG_SELINUX
 #include <selinux/selinux.h>  
 #endif