Add mg, a gnu-emacs like fork of microemacs (#13812)

* Add mg, a gnu-emacs like fork of microemacs

* Use Package, since not really an Autotools package

Switch from AutotoolsPackage to Package.  Even though mg has a
configure script, it's not really an Autotools package.

* Need to also provide --prefix to configure

var/spack/repos/builtin/packages/mg/package.py

+# Copyright 2013-2019 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)
+
+from spack import *
+
+
+class Mg(Package):
+    """Mg is intended to be a small, fast, and portable editor for people
+    who can't (or don't want to) run emacs for one reason or another,
+    or are not familiar with the vi editor. It is compatible with
+    emacs because there shouldn't be any reason to learn more editor
+    types than emacs or vi."""
+
+    homepage = "https://github.com/ibara/mg"
+    url      = "https://github.com/ibara/mg/archive/mg-6.6.tar.gz"
+
+    version('6.6', sha256='e8440353da1a52ec7d40fb88d4f145da49c320b5ba31daf895b0b0db5ccd0632')
+
+    depends_on('ncurses')
+
+    phases = ['configure', 'build', 'install']
+
+    def configure(self, spec, prefix):
+        configure = Executable('./configure')
+        args = [
+            '--mandir={0}'.format(self.prefix.man),
+            '--prefix={0}'.format(self.prefix),
+        ]
+        configure(*args)
+
+    def build(self, spec, prefix):
+        make()
+
+    def install(self, spec, prefix):
+        make('install')