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.

zconf.h

@@ -266,11 +266,11 @@
 #  endif
 #endif
 
-#ifndef ON /* function prototypes for stdarg */
+#ifndef Z_ARG /* function prototypes for stdarg */
 #  if defined(STDC) || defined(Z_HAVE_STDARG_H)
-#    define ON(args)  args
+#    define Z_ARG(args)  args
 #  else
-#    define ON(args)  ()
+#    define Z_ARG(args)  ()
 #  endif
 #endif
 

zconf.h.cmakein

@@ -268,11 +268,11 @@
 #  endif
 #endif
 
-#ifndef ON /* function prototypes for stdarg */
+#ifndef Z_ARG /* function prototypes for stdarg */
 #  if defined(STDC) || defined(Z_HAVE_STDARG_H)
-#    define ON(args)  args
+#    define Z_ARG(args)  args
 #  else
-#    define ON(args)  ()
+#    define Z_ARG(args)  ()
 #  endif
 #endif
 

zconf.h.in

@@ -266,11 +266,11 @@
 #  endif
 #endif
 
-#ifndef ON /* function prototypes for stdarg */
+#ifndef Z_ARG /* function prototypes for stdarg */
 #  if defined(STDC) || defined(Z_HAVE_STDARG_H)
-#    define ON(args)  args
+#    define Z_ARG(args)  args
 #  else
-#    define ON(args)  ()
+#    define Z_ARG(args)  ()
 #  endif
 #endif
 

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