Using msys2 for mxe-octave

[JohnD]

Now that Octave 4.4.1 is here, I've been playing around with getting octave
to work using msys2 rather than the old msys, and have pushed up a patch:

https://savannah.gnu.org/patch/index.php?9675

It provides a --with-msys2 option to the mxe-octave configure script, which
will build the installer/tarball with the msys2 system utilities and
libraries rather than the msys ones.

In addition it has pacman (the msys2 package manager) installed and allows
downloading of other msys2 packages easily from within the installed
application.

Apart from the msys2 packages that get installed, the octave packages and
build tools will get installed in <base>/mingw32 or <base>/mingw32, rather
than the base directory which is where the native application like gcc etc
would be installed in msys2.

I haven't pushed it to mxe-octave as yet, waiting on whether it's a good
thing to have and other comments.