webxr: don't use CMake 3.13 functionality if we support 3.4.
webxr: don't use CMake 3.13 functionality if we support 3.4.

And fix the path also.

mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples