only compile 106 if imgui is selected

Former-commit-id: fcc97bc619e417f50162b3fa4c26510a265c3ac9

tutorial/CMakeLists.txt

@@ -53,7 +53,9 @@ if(TUTORIALS_CHAPTER1)
   add_subdirectory("103_Events")
   add_subdirectory("104_Colors")
   add_subdirectory("105_Overlays")
-  add_subdirectory("106_ViewerMenu")
+  if(LIBIGL_WITH_OPENGL_GLFW_IMGUI)
+    add_subdirectory("106_ViewerMenu")
+  endif()
   add_subdirectory("107_MultipleMeshes")
 endif()