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.

zconf.h

@@ -214,6 +214,12 @@
 #  endif
 #endif
 
+#if defined(ZLIB_CONST) && !defined(z_const)
+#  define z_const const
+#else
+#  define z_const
+#endif
+
 /* Some Mac compilers merge all .h files incorrectly: */
 #if defined(__MWERKS__)||defined(applec)||defined(THINK_C)||defined(__SC__)
 #  define NO_DUMMY_DECL