[CMAKE] Fixes FindGrape if no X11 or OpenGL is present.

Previously, both X11 and OpenGL were required when searching
for them using find_package. This resulted in a fatal error
during the configure stage if either of them was not found.
As s result dune-grid was not usable without X11 and OpenGL.

This patches fixes this situation by making X11 and OpenGL
optional. Thus configure succeeds even without them as it
should.

These are the patches 283f0a1 6c8381b cherry-picked manually
from release/2.2-cmake.