* vtk: add variants for Xdmf IO support, ffmpeg support, and MPI
* vtk: depends on boost when Xdmf support is enabled
* vtk: add backported patch for python3.7
* vtk: New policy only for cmake >= 3.12
* mesa: add py-argparse dependency (for build)
* lz4: use MOREFLAGS instead of LIBS to add -lrt
- remove the old LGPL license headers from all files in Spack
- add SPDX headers to all files
- core and most packages are (Apache-2.0 OR MIT)
- a very small number of remaining packages are LGPL-2.1-only
* fix missing -lrt for Centos6, issue: #7552
* fix missing -lrt for Centos6, issue: #7552
* fix using make() as opposed to patching
* remove patch, as the fix is provided withing package.py instead
* make flake8 happy
