[m-rev.] diff: fix source distribution builds

[Julien Fischer]

On Mon, 13 Jan 2020, Zoltan Somogyi wrote:

>
> 2020-01-12 23:54 GMT+11:00 Julien Fischer<jfischer at opturion.com>:
>>      Restore the .PHONY directives on the Mercury.modules target as their
>>      absence is causing the source distribution builds to fail.
>
> What was the symptom?

The Mercury.modules file not being created and compilation falling over
because some module names in those directories don't match file names.

Julien.