CMAKE: add unity builds support

5 jobs for add-unity-builds (queued for 50 seconds)
Status Name Job ID Coverage
  External
passed cdash-buildbot

pending vtk-m-dragnipur-osx-shared-debug+32bit_ids+64bit_coords+clang+tbb
pending vtk-m-dragnipur-osx-static-release+64bit_coords+clang+tbb
pending vtk-m-osheim-windows-shared-debug+ninja-vs-2017+tbb
pending vtk-m-osheim-windows-static-release+ninja-vs-2017+tbb