(CMake): Remove the warning about LEGACY and GLVND libraries for OpenGL

What does this MR do?

Resolve Warning about findOpenGL on Linux, by setting the cmake policy CMP0072 to NEW.

Closes #351 (closed)

How to test it?

1. Configure sight with cmake (> 3.11) on Linux
2. Check that the findOpenGL warning doesn't pop. (see #351 (closed) for more information).

Associated Issues/Merge Requests

• Issues
  • Closes #351 (closed)