- 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
* pnfft: new package
* Convert some packages with overridden install from AutotoolsPackage to Package
* pnfft: fix URL
* Switch FFTW-derived packages back to AutotoolsPackage
* Disable unneeded build phases in FFTW and derived packages
* Separate build phases for FFTW and derived packages
* Fix broken merge
* fftw: pfft_patches for 3.3.6
* fftw: address @adamjstewart’s review comments
