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

[Zoltan Somogyi]

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

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

Zoltan.
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