Add #define ZLIB_CONST option to use const in the z_stream interface.

This permits compilers to check for the proper treatment of next_in and
msg in the z_stream structure.  This is an option instead of the default
in order to preserve backward compatibility.  Some applications make use
of the z_stream structure outside of zlib, and perform operations such
as free(strm->next_in), which would not be permitted when next_in is
const.  The #define ZLIB_CONST needs to precede the #include "zlib.h">,
in order to make next_in and msg const pointers in the z_stream type.

zlib.h

@@ -83,7 +83,7 @@ typedef void   (*free_func)  OF((voidpf opaque, voidpf address));
 struct internal_state;
 
 typedef struct z_stream_s {
-    Bytef    *next_in;  /* next input byte */
+    z_const Bytef *next_in;     /* next input byte */
     uInt     avail_in;  /* number of bytes available at next_in */
     uLong    total_in;  /* total number of input bytes read so far */
 
@@ -91,7 +91,7 @@ typedef struct z_stream_s {
     uInt     avail_out; /* remaining free space at next_out */
     uLong    total_out; /* total number of bytes output so far */
 
-    char     *msg;      /* last error message, NULL if no error */
+    z_const char *msg;  /* last error message, NULL if no error */
     struct internal_state FAR *state; /* not visible by applications */
 
     alloc_func zalloc;  /* used to allocate the internal state */