- 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
* Clean up the gcc package
* Add preliminary testing support
* Older versions of GCC do not depend on MPC
* DejaGnu 1.4.4 cannot be built in parallel
* Fix GCC test dir
* Add two more deps required to run the testsuite