Change ON macro to Z_ARG to avoid application conflicts.

Using "ON" was a dumb idea, since it is common to have macros with
names like ON and OFF.  In fact, defining the OF macro back in 1995
was a bad idea, but now we're stuck with it.  Attempts to rename OF
to something else breaks many applications.

zlib.h

@@ -1298,7 +1298,7 @@ ZEXTERN int ZEXPORT gzwrite OF((gzFile file,
    error.
 */
 
-ZEXTERN int ZEXPORTVA gzprintf ON((gzFile file, const char *format, ...));
+ZEXTERN int ZEXPORTVA gzprintf Z_ARG((gzFile file, const char *format, ...));
 /*
      Converts, formats, and writes the arguments to the compressed file under
    control of the format string, as in fprintf.  gzprintf returns the number of