build: install with the correct permissions

This ensures that files that are meant to be executable are installed as
such in the runtime area.

There is some trickiness here because the install() command will create
the parent directories, but it does so with the user's umask.  Instead,
we reach a compromise.  Everything in share/nvim should be installed so
that it's world readable.  In the case that the install prefix doesn't
exist, we'll create it with world readable privileges.  If it exists
already, we'll leave it alone.