- 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
Adds Google's new cpu_features library.
Install was added recently, so don't take the `0.1.0` release but
the current `master` until a new version is published.