* Cleanup list of offered versions (only the latest update release of each minor version is kept: 1.4.4, 1.5.2, 1.6.6, 1.7.1); introduced master revision (via Git). Slightly more selective installation of the documentation; copy README.md and LICENSE files to documentation folder. Removed outdated build options (make_args); generally stay with LIBXSMM's defaults (general purpose build). Introduced one build variant ("debug").
* Incorporated comments of PR #3371 (https://github.com/LLNL/spack/pull/3371).
* Fixed flake8 complaints.
* Made the full list of versions available starting with version 1.4 (there are earlier releases, however those have been released before the Spack specification became available; original spec. also started with 1.4).
* Made DBG and TRACE actually two separate arguments.
