COMP: Fix deprecated interface warning when building designer plugin with Qt5

This commit fixes the following warning:

```
In file included from /path/to/CTK-build/CTK-build/Libs/Widgets/Plugins/../../../../../CTK/Libs/Widgets/Plugins/ctkWidgetsPlugins.h:27:0,
                 from /path/to/CTK-build/CTK-build/Libs/Widgets/Plugins/moc_ctkWidgetsPlugins.cpp:9:
/path/to/Qt5.9.1/5.9.1/gcc_64/include/QtDesigner/QDesignerCustomWidgetCollectionInterface:4:4: warning: #warning Header <QtDesigner/QDesignerCustomWidgetCollectionInterface> is deprecated. Please include <QtUiPlugin/QDesignerCustomWidgetCollectionInterface> instead. [-Wcpp]
 #  warning Header <QtDesigner/QDesignerCustomWidgetCollectionInterface> is deprecated. Please include <QtUiPlugin/QDesignerCustomWidgetCollectionInterface> instead.
    ^
```

CMake/ctkMacroBuildQtPlugin.cmake

@@ -136,8 +136,12 @@ macro(ctkMacroBuildQtPlugin)
   string(REGEX REPLACE "(.*)Plugin[s]?" "\\1" label ${lib_name})
 
   # Apply properties to the library target.
+  set(compile_flags "-DQT_PLUGIN")
+  if(CTK_QT_VERSION VERSION_GREATER "4")
+    set(compile_flags "${compile_flags} -DHAVE_QT5")
+  endif()
   set_target_properties(${lib_name}  PROPERTIES
-    COMPILE_FLAGS "-DQT_PLUGIN"
+    COMPILE_FLAGS "${compile_flags}"
     LIBRARY_OUTPUT_DIRECTORY "${CMAKE_LIBRARY_OUTPUT_DIRECTORY}/${MY_PLUGIN_DIR}"
     LABELS ${label}
     )