store settings to different locations for different Qt versions

tools/lammps-gui/lammpsgui.cpp

@@ -84,9 +84,13 @@ LammpsGui::LammpsGui(QWidget *parent, const char *filename) :
     // use $HOME if we get dropped to "/" like on macOS
     if (current_dir == "/") current_dir = QDir::homePath();
 
+#define stringify(x) myxstr(x)
+#define myxstr(x) #x
     QCoreApplication::setOrganizationName("The LAMMPS Developers");
     QCoreApplication::setOrganizationDomain("lammps.org");
-    QCoreApplication::setApplicationName("LAMMPS GUI");
+    QCoreApplication::setApplicationName("LAMMPS GUI - QT" stringify(QT_VERSION_MAJOR));
+#undef stringify
+#undef myxstr
 
     // restore and initialize settings
     QSettings settings;