Improve setup for 'dlg' library.

* autogen.sh (copy_submodule_file), builds/toplevel.mk: Redirect
stderr to `/dev/null`.

* builds/toplevel.mk: Move code block to handle 'dlg' stuff into
`check_platform` conditional.
Also fix wildcard expressions for guarding `git submodule` commands.
Also make file copying work with non-Unix platforms (untested).