define ZEXPORT as empty if the system zlib.h doesn't define it (fixes bug 583681)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_4_BRANCH@17278 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/unzip.h

@@ -78,6 +78,13 @@ extern "C" {
 #   define OF(args) args
 #endif
 
+/*
+   And on some systems ZEXPORT is not defined neither -- work around this too
+ */
+#ifndef ZEXPORT
+#   define ZEXPORT
+#endif
+
 #if defined(STRICTUNZIP) || defined(STRICTZIPUNZIP)
 /* like the STRICT of WIN32, we define a pointer that cannot be converted
     from (void*) without cast */