[m-rev.] for review: Install stub mdb_doc if makeinfo/info are not available.

[Julien Fischer]

On Mon, 27 May 2019, Peter Wang wrote:

> doc/mdb_doc_stub.txt:
>    Add a stub file.
>
> doc/Mmakefile
>    Copy the stub file if mdb_doc cannot be generated.

That's fine.

Julien.