[m-rev.] for post-commit review: improve configure test for broken Xcode 11

[Julien Fischer]

Hi Zoltan,

On Sat, 27 Jun 2020, Zoltan Somogyi wrote:

> Consider the attached diff to be my review of your diff.
> I await your review of my diff.

That looks fine; nice catch on the configure script change.

> We should also consider deleting all references to MacOS versions
> and compiler versions that are older than a given threshold.
> For example, the paragraphs that talk about e.g. 10.5 are
> very likely to be of no use to anyone anymore.

Post 20.06, I intend do to do just that.  We should also drop support
for the powerpc-apple-darwin architecture entirely.

> Why should *we* talk about how to install Mercury on MacOS versions
> that Apple doesn't support anymore?

In my experience Apple hardware tends to remain useful much longer than
their (very short) support for OS releases, so some some discussion of
earlier versions is useful.

Julien.