Fix duplicate about entries in mac app menu

Clearing the menu bar does not remove the actions from the app menu,
but setting the action role allows Qt to take care of duplicates.

src/Gui/CommandStd.cpp

@@ -203,10 +203,14 @@ Action * StdCmdAbout::createAction(void)
         this->className(), sStatusTip).arg(exe));
     pcAction->setWhatsThis(QLatin1String(sWhatsThis));
     pcAction->setIcon(QApplication::windowIcon());
-    pcAction->setShortcut(QString::fromLatin1(sAccel));
-    //Prevent Qt from using AboutRole -- fixes issue #0001485
+    pcAction->setShortcut(QString::fromLatin1(sAccel));
+#if QT_VERSION > 0x050000
+    // Needs to have AboutRole set to avoid duplicates if adding the about action more than once on macOS
+    pcAction->setMenuRole(QAction::AboutRole);
+#else
+    // With Qt 4.8, having AboutRole set causes it to disappear when readding it: issue #0001485
     pcAction->setMenuRole(QAction::ApplicationSpecificRole);
-
+#endif
     return pcAction;
 }
 

src/Gui/MenuManager.cpp

@@ -192,7 +192,7 @@ void MenuManager::setup(MenuItem* menuItems) const
 
     QMenuBar* menuBar = getMainWindow()->menuBar();
 
-#ifdef FC_OS_MACOSX
+#if defined(FC_OS_MACOSX) && QT_VERSION >= 0x050900
     // Unknown Qt macOS bug observed with Qt >= 5.9.4 causes random crashes when viewing reused top level menus.
     menuBar->clear();
 #endif