Make is a required dependency of Spack (#10386)

Update documentation on getting started to mention that a "make"
executable is required to build packages with Spack.

lib/spack/docs/getting_started.rst

@@ -16,10 +16,11 @@ Prerequisites
 Spack has the following minimum requirements, which must be installed
 before Spack is run:
 
-1. Python 2 (2.6 or 2.7) or 3 (3.4 - 3.7)
-2. A C/C++ compiler
-3. The ``git`` and ``curl`` commands.
-4. If using the ``gpg`` subcommand, ``gnupg2`` is required.
+#. Python 2 (2.6 or 2.7) or 3 (3.4 - 3.7) to run Spack
+#. A C/C++ compiler for building
+#. The ``make`` executable for building
+#. The ``git`` and ``curl`` commands for fetching
+#. If using the ``gpg`` subcommand, ``gnupg2`` is required
 
 These requirements can be easily installed on most modern Linux systems;
 on Macintosh, XCode is required.  Spack is designed to run on HPC