Workaround Qt5 bug that fails to remove separator at end of Help menu.

When the About menu item is relocated to Application Menu on
Mac OS X / macOS the separator above it should be removed. While
Qt4 removes the separator, Qt5 does not remove the separator.

For details see:
 * <LMMS#3345>
 * <https://bugreports.qt.io/browse/QTBUG-40071>

src/gui/MainWindow.cpp

@@ -399,7 +399,16 @@ void MainWindow::finalize()
 					tr( "What's This?" ),
 					this, SLOT( enterWhatsThisMode() ) );
 
+//
+// Conditional compilation to workaround Qt5 bug that fails to remove
+// separator at end of menu when the About menu item is relocated to
+// Application Menu on Mac OS X / macOS. For details see:
+//  * <https://github.com/LMMS/lmms/issues/3345>
+//  * <https://bugreports.qt.io/browse/QTBUG-40071>
+//
+#if !(defined(LMMS_BUILD_APPLE) && (QT_VERSION >= 0x050000))
 	help_menu->addSeparator();
+#endif
 	help_menu->addAction( embed::getIconPixmap( "icon" ), tr( "About" ),
 				  this, SLOT( aboutLMMS() ) );