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.

compress.c

@@ -29,7 +29,7 @@ int ZEXPORT compress2 (dest, destLen, source, sourceLen, level)
     z_stream stream;
     int err;
 
-    stream.next_in = (Bytef*)source;
+    stream.next_in = (z_const Bytef *)source;
     stream.avail_in = (uInt)sourceLen;
 #ifdef MAXSEG_64K
     /* Check for source > 64K on 16-bit machine: */