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.