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.

gzread.c

@@ -58,7 +58,8 @@ local int gz_avail(state)
         return -1;
     if (state->eof == 0) {
         if (strm->avail_in) {       /* copy what's there to the start */
-            unsigned char *p = state->in, *q = strm->next_in;
+            unsigned char *p = state->in;
+            unsigned const char *q = strm->next_in;
             unsigned n = strm->avail_in;
             do {
                 *p++ = *q++;