Fix undefined macro issue when compiling with zlib.
https://bugs.gentoo.org/753950

From upstream:
https://sourceforge.net/p/ucon64/svn/2817/
https://sourceforge.net/p/ucon64/svn/2819/
--- a/libdiscmage/ioapi.h
+++ b/libdiscmage/ioapi.h
@@ -35,4 +35,7 @@
 extern "C" {
 #endif
+#ifndef OF
+#define OF(x) x
+#endif
 
 typedef voidpf (ZCALLBACK *open_file_func) OF((voidpf opaque, const char* filename, int mode));
--- a/misc/ioapi.h
+++ b/misc/ioapi.h
@@ -35,4 +35,7 @@
 extern "C" {
 #endif
+#ifndef OF
+#define OF(x) x
+#endif
 
 typedef voidpf (ZCALLBACK *open_file_func) OF((voidpf opaque, const char* filename, int mode));