Qt4/Qt5 neutral changes:

+ replace Q_WS_WIN with Q_OS_WIN
+ replace Q_WS_X11 with Q_OS_LINUX
+ replace Q_WS_MACX with Q_OS_MACX
+ set explicit cast to HWND

src/Gui/Application.cpp

@@ -1627,7 +1627,7 @@ void Application::runApplication(void)
     else if (version & QGLFormat::OpenGL_Version_None)
         Base::Console().Log("No OpenGL is present or no OpenGL context is current\n");
 
-#if !defined(Q_WS_X11)
+#if !defined(Q_OS_LINUX)
     QIcon::setThemeSearchPaths(QIcon::themeSearchPaths() << QString::fromLatin1(":/icons/FreeCAD-default"));
     QIcon::setThemeName(QLatin1String("FreeCAD-default"));
 #endif