Merge topic 'FindGLUT-include-dirs'

6fda93faf8 FindGLUT: Provide modern-named GLUT_INCLUDE_DIRS result variable
641b3f4101 FindGLUT: Modernize documentation layout
924b2999a1 FindGLUT: Move mark_as_advanced calls closer to find calls

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !6820