[MacPorts] #62805: et @6.1.7: builds failing for all macOS releases earlier than 10.15

[MacPorts]

#62805: et @6.1.7: builds failing for all macOS releases earlier than 10.15
----------------------+----------------------
  Reporter:  mascguy  |      Owner:  l2dy
      Type:  defect   |     Status:  assigned
  Priority:  Normal   |  Milestone:
 Component:  ports    |    Version:
Resolution:           |   Keywords:
      Port:  et       |
----------------------+----------------------

Comment (by ryandesign):

 MacPorts base does "enforce" all values of `compiler.cxx_standard` to the
 extent that it makes sure that a compiler that it believes is compatible
 with that standard is used. It is of course possible that MacPorts base
 contains inaccurate information about which compilers support which C++
 standard. See for example [e641f42a9f6d93e1768bd91785d19467c30ddb02
 /macports-base].

 The error I see in the log is:
 {{{
 error: 'exists' is unavailable: introduced in macOS 10.15
 }}}
 In that case, no amount of using different compilers will help if the
 functionality that is required is not present in (the C++ standard library
 of) earlier OS versions.

-- 
Ticket URL: <https://trac.macports.org/ticket/62805#comment:3>
MacPorts <https://www.macports.org/>
Ports system for macOS