Clean up the usage of z_const and respect const usage within zlib.

This patch allows zlib to compile cleanly with the -Wcast-qual gcc
warning enabled, but only if ZLIB_CONST is defined, which adds
const to next_in and msg in z_stream and in the in_func prototype.
A --const option is added to ./configure which adds -DZLIB_CONST
to the compile flags, and adds -Wcast-qual to the compile flags
when ZLIBGCCWARN is set in the environment.

zutil.c

@@ -14,7 +14,7 @@
 struct internal_state      {int dummy;}; /* for buggy compilers */
 #endif
 
-const char * const z_errmsg[10] = {
+z_const char * const z_errmsg[10] = {
 "need dictionary",     /* Z_NEED_DICT       2  */
 "stream end",          /* Z_STREAM_END      1  */
 "",                    /* Z_OK              0  */