vtkWithQt4: actually build QVTK libs/headers

[bcdarwin]

Motivation for this change

• VTK_USE_QT is obsolete and didn't have the intended effect. (For instance, you may verify that QVTKWidgets.h is now present in the output, unlike previously.)

cc @knedlsepp @veprbl

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• [NA] Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review-may-2019/3032/148

[veprbl]

@GrahamcOfBorg build vtkWithQt4 vtkWithQt4.passthru.tests

[bcdarwin]

The Darwin build hasn't run yet but Qt4 doesn't evaluate on Darwin anyway.