Also update installation to install headers via `install(FILE_SET)`.

This will allow for CMake to understand what files belong to which
target and avoid unnecessary dependencies.

Given that file sets require a new CMake, abstract away the version
detection.

Introduced in CMake 3.11, so we can assume it is available.

There's nothing wrappable here anyways.

They're not *that* special.

The `IMPLICIT_DEPENDS` argument is only supported in Makefiles
generators while `DEPFILE` is supported "everywhere" now. It now also
includes `hierarchy` files that are read during generation.

The `IMPLICIT_DEPENDS` argument is only supported in Makefiles
generators while `DEPFILE` is supported "everywhere" now. It now also
includes `hierarchy` files that are read during generation.

This will cause the proper hierarchy files to be regenerated when
transitive headers are used in their generation.

For some reason, `octree`'s `octree/octree` header ends up causing a
dependency cycle of `vtkoctree-spdx` onto itself. Make paths absolute to
avoid this problem.

Also quote the output file path.