[m-rev.] for post-commit improvement: warn about Xcode 11

[Julien Fischer]

On Thu, 7 May 2020, Zoltan Somogyi wrote:

> Julien, can you please look into making the printing of the warning
> conditional on either the Xcode version being 11.x with x < 4,
> or on the corresponding test on the clang version, whichever
> is appropriate? I think you know MacOS programming tools
> much better than I do.

Shall do.

Julien.