Update for zlib header changes for zlib > 1.2.5

Gentoo bug: https://bugs.gentoo.org/show_bug.cgi?id=383431
R bug: https://bugs.r-project.org/bugzilla3/show_bug.cgi?id=14951

Patch by Michael Schreckenbauer

--- src/main/unzip.h.orig	2011-09-24 19:35:53.000000000 -0700
+++ src/main/unzip.h	2011-09-24 19:37:06.000000000 -0700
@@ -58,6 +58,10 @@
 #define ZLIB_FILEFUNC_SEEK_END (2)
 #define ZLIB_FILEFUNC_SEEK_SET (0)
 
+#ifndef OF
+#define OF(x) x
+#endif
+
 #define ZLIB_FILEFUNC_MODE_READ      (1)
 #define ZLIB_FILEFUNC_MODE_WRITE     (2)
 #define ZLIB_FILEFUNC_MODE_READWRITEFILTER (3)