Branch with the meson build-system (#8467)

* Branch with the meson build-system

* Fix build_environment for dual loads and add create code

* Add documentation

* Fixed option list

* Update build_system_guess for meson

* Fixed documentation errors

* Added meson to build and configure and updated documentation

* fix typos

lib/spack/docs/build_systems.rst

@@ -33,6 +33,7 @@ on these ideas for each distinct build system that Spack supports:
 
    build_systems/autotoolspackage
    build_systems/cmakepackage
+   build_systems/mesonpackage
    build_systems/qmakepackage
 
 .. toctree::

lib/spack/docs/build_systems/mesonpackage.rst

@@ -0,0 +1,85 @@
+.. _mesonpackage:
+
+------------
+MesonPackage
+------------
+
+Much like Autotools and CMake, Meson is a build system.  But it is
+meant to be both fast and as user friendly as possible.  GNOME's goal
+is to port modules to use the Meson build system.
+
+^^^^^^
+Phases
+^^^^^^
+
+The ``MesonPackage`` base class comes with the following phases:
+
+#. ``meson`` - generate ninja files
+#. ``build`` - build the project
+#. ``install`` - install the project
+
+By default, these phases run:
+
+.. code-block:: console
+
+   $ mkdir spack-build
+   $ cd spack-build
+   $ meson .. --prefix=/path/to/installation/prefix
+   $ ninja
+   $ ninja test  # optional
+   $ ninja install
+
+
+Any of these phases can be overridden in your package as necessary.
+There is also a ``check`` method that looks for a ``test`` target
+in the build file. If a ``test`` target exists and the user runs:
+
+.. code-block:: console
+
+   $ spack install --test=root <meson-package>
+
+
+Spack will run ``ninja test`` after the build phase.
+
+^^^^^^^^^^^^^^^
+Important files
+^^^^^^^^^^^^^^^
+
+Packages that use the Meson build system can be identified by the
+presence of a ``meson.build`` file. This file declares things
+like build instructions and dependencies.
+
+^^^^^^^^^^^^^^^^^^^^^^^^^
+Build system dependencies
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+At the bare minimum, packages that use the Meson build system need
+``meson`` and ```ninja``` dependencies. Since this is always the case,
+the ``MesonPackage`` base class already contains:
+
+.. code-block:: python
+
+   depends_on('meson', type='build')
+   depends_on('ninja', type='build')
+
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+Passing arguments to meson
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If you need to pass any arguments to the ``meson`` call, you can
+override the ``meson_args`` method like so:
+
+.. code-block:: python
+
+   def meson_args(self):
+       return ['--default-library=both']
+
+
+This method can be used to pass flags as well as variables.
+
+^^^^^^^^^^^^^^^^^^^^^^
+External documentation
+^^^^^^^^^^^^^^^^^^^^^^
+
+For more information on the Meson build system, see:
+https://mesonbuild.com/index.html