Add support for racket packages (#27564)

- Add variants for various common build flags, including support for both versions of the Racket VM environment.
- Prevent `-j` flags to `make`, which has been known to cause problems with Racket builds.
- Prefer the minimal release to improve install times. Bells and whistles carry their own runtime dependencies and should be installed via `raco`. An enterprising user may even create a `RacketPackage` class to make spack aware of `raco` installed packages.
- Match the official version numbering scheme.

lib/spack/docs/build_systems.rst

@@ -51,6 +51,7 @@ on these ideas for each distinct build system that Spack supports:
    build_systems/perlpackage
    build_systems/pythonpackage
    build_systems/rpackage
+   build_systems/racketpackage
    build_systems/rubypackage
 
 .. toctree::

lib/spack/docs/build_systems/racketpackage.rst

@@ -0,0 +1,46 @@
+.. Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+   Spack Project Developers. See the top-level COPYRIGHT file for details.
+
+   SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+.. _racketpackage:
+
+-------------
+RacketPackage
+-------------
+
+Much like Python, Racket packages and modules have their own special build system.
+To learn more about the specifics of Racket package system, please refer to the
+`Racket Docs <https://docs.racket-lang.org/pkg/cmdline.html>`_.
+
+^^^^^^
+Phases
+^^^^^^
+
+The ``RacketPackage`` base class provides an ``install`` phase that
+can be overridden, corresponding to the use of:
+
+.. code-block:: console
+
+   $ raco pkg install
+
+^^^^^^^
+Caveats
+^^^^^^^
+
+In principle, ``raco`` supports a second, ``setup`` phase; however, we have not
+implemented this separately, as in normal circumstances, ``install`` also handles
+running ``setup`` automatically.
+
+Unlike Python, Racket currently on supports two installation scopes for packages, user
+or system, and keeps a registry of installed packages at each scope in its configuration files.
+This means we can't simply compose a "``RACKET_PATH``" environment variable listing all of the
+places packages are installed, and update this at will.
+
+Unfortunately this means that all currently installed packages which extend Racket via ``raco pkg install``
+are accessible whenever Racket is accessible.
+
+Additionally, because Spack does not implement uninstall hooks, uninstalling a Spack  ``rkt-`` package
+will have no effect on the ``raco`` installed packages visible to your Racket installation.
+Instead, you must manually run ``raco pkg remove`` to keep the two package managers in a mutually
+consistent state.