Make testcases build, prevent windows symbol collision

src/qt/bitcoin.cpp

@@ -42,7 +42,7 @@ static void ThreadSafeMessageBox(const std::string& message, const std::string&
     // Message from network thread
     if(guiref)
     {
-        bool modal = (style & MF_MODAL);
+        bool modal = (style & CClientUIInterface::MODAL);
         // in case of modal message, use blocking connection to wait for user to click OK
         QMetaObject::invokeMethod(guiref, "error",
                                    modal ? GUIUtil::blockingGUIThreadConnection() : Qt::QueuedConnection,