Gentoo's zlib.h does s/OF/_Z_OF/, so define OF if missing.
https://bugs.gentoo.org/383179
--- a/src/unzip.h
+++ b/src/unzip.h
@@ -59,4 +59,7 @@
 #endif
 
+#ifndef OF
+#define OF(x) x
+#endif
 
 #define UNZ_OK                                  (0)