[MacPorts] #69975: re2 @2024-04-01_2: fails to build on macOS 10.12/Xcode 9.2 due to <optional> not found in abseil headers

[MacPorts]

#69975: re2 @2024-04-01_2: fails to build on macOS 10.12/Xcode 9.2 due to
<optional> not found in abseil headers
----------------------+-------------------------
  Reporter:  andlabs  |      Owner:  herbygillot
      Type:  defect   |     Status:  assigned
  Priority:  Normal   |  Milestone:
 Component:  ports    |    Version:  2.9.3
Resolution:           |   Keywords:
      Port:  re2      |
----------------------+-------------------------

Comment (by FlyingSamson):

 From this [https://gist.github.com/yamaya/2924292 apple clang vs Xcode
 version list], I would expect that your `/usr/bin/clang++` is of version
 9.0.0.

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Ticket URL: <https://trac.macports.org/ticket/69975#comment:4>
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