Fix build problem for when not compiled with debugging.

dbug/dbug.c:
  Don't try to force DBUG on.
  
  Conditionally compile this file, as the headers we load may change the 
  value of DBUG_OFF /after/ we set it.

dbug/dbug.c

@@ -71,9 +71,7 @@
  *
  */
 
-#ifdef DBUG_OFF
-#undef DBUG_OFF
-#endif
+
 #include <my_global.h>
 #include <m_string.h>
 #include <errno.h>
@@ -81,6 +79,10 @@
 #include <process.h>
 #endif
 
+
+#ifndef DBUG_OFF
+
+
 /*
  *            Manifest constants which may be "tuned" if desired.
  */
@@ -313,6 +315,7 @@ static unsigned long Clock(void);
 #define ChangeOwner(cs,name)
 #endif
 
+
 /*
 ** Macros to allow dbugging with threads
 */
@@ -2351,3 +2354,5 @@ va_list ap;
 }
 
 #endif  /* NO_VARARGS */
+
+#endif