[m-rev.] for possible review: make ':- external' a pragma

[Julien Fischer]

On Wed, 22 Jul 2015, Zoltan Somogyi wrote:

> Is anyone interested in reviewing this? If yes, I will wait for the review,
> and work on something else. If not, I will commit it and build on it.
>

The diff looks fine.  Are you intending to remove the ':- external'
variant at some point?

Cheers,
Julien.