Enable Nanogui for Mac.

Former-commit-id: ed18f6e764ef8cf185814df02fd06494121e0db5

.travis.yml

@@ -63,5 +63,5 @@ matrix:
         - cd tutorial
         - mkdir build
         - cd build
-        - cmake -DLIBIGL_USE_STATIC_LIBRARY=ON -DLIBIGL_WITH_NANOGUI=OFF -DLIBIGL_WITH_EMBREE=OFF ../
+        - cmake -DLIBIGL_USE_STATIC_LIBRARY=ON -DLIBIGL_WITH_EMBREE=OFF ../
         - make -j 2