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

LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,iulian787/spack