VTK actually links to libx11.so and libxt is not needed.

The build works without it and gnuplot does not seem to be used in the
entire project except for building VTK documentation:

https://gitlab.kitware.com/search?search=gnuplot&nav_source=navbar&project_id=13&group_id=16&search_code=true&repository_ref=master

Upstream dropped glew and opengl modules and loads the appropriate
OpenGL library at runtime.

https://gitlab.kitware.com/vtk/vtk/-/merge_requests/11482
https://docs.vtk.org/en/latest/release_details/9.4.html#rendering

The vtkzfp module is marked as internal (vtk_module_third_party_internal())
so the build cannot use external zfp without patching.

https://docs.vtk.org/en/latest/api/cmake/ModuleSystem.html#third-party

RapidJSON is unmaintained (last release in 2016) and VTK bundles its own
patched version. The Arch package does not seem to be usable any time
soon.

These are needed for building VTK documentation, but it is disabled
since 2017.

The package currently does not need git for source fetching nor
patching.

It is not required for building VTK, but when building downstream
projects with Clang.

Fixes: #3