Move messages.c to libbb. Make each string in messages.c be its own .o file.

This way, we can new get rid of all that tedious #define rubbish we used to
need to enable specific messages.  This way is enormously simpler, and as a
bonus also ends up saving us 96 bytes.
 -Erik

coreutils/ln.c

@@ -28,9 +28,6 @@
 #include <errno.h>
 #include <unistd.h>
 #include "busybox.h"
-#define BB_DECLARE_EXTERN
-#define bb_need_not_a_directory
-#include "messages.c"
 
 
 static const int LN_SYMLINK = 1;