More fixes for gzopen_w().

Also need to #include <stddef.h> for zlib.h, and need to workaround
the inability to use wide characters in constructed error messages
with zlib's interface.

zconf.h

@@ -402,6 +402,10 @@ typedef uLong FAR uLongf;
 #  endif
 #endif
 
+#ifdef _WIN32
+#  include <stddef.h>           /* for wchar_t */
+#endif
+
 /* a little trick to accommodate both "#define _LARGEFILE64_SOURCE" and
  * "#define _LARGEFILE64_SOURCE 1" as requesting 64-bit operations, (even
  * though the former does not conform to the LFS document), but considering