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

[Zoltan Somogyi]

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.

Zoltan.
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