Disable examples by default

Disable building examples by default since:

* They require GLFW, at least for me it's confusing to be presented with this requirement by cmake if I just want to build the library (and that's the most common use case of globjects, I think. Few people care about examples).
* At least one example (WorldInHandNavigation, iirc, including glm/gtx/constants.hpp) uses a header from glm  0.9.4, but the official requirement of globjects is glm >= 0.9.5