[MacPorts] #60853: port mysql8 @8.0.21 fails to build on Mac OS 10.9 mavericks

[MacPorts]

#60853: port mysql8 @8.0.21 fails to build on Mac OS 10.9 mavericks
---------------------+-----------------------
  Reporter:  wadjei  |      Owner:  (none)
      Type:  defect  |     Status:  new
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:  2.6.2
Resolution:          |   Keywords:  mavericks
      Port:  mysql8  |
---------------------+-----------------------

Comment (by kencu):

 we will be seeing more of these kinds of errors:
 {{{
 mysql-8.0.21/storage/temptable/include/temptable/kv_store.h:76:50: error:
 'shared_timed_mutex' is unavailable: introduced in macOS 10.12
       std::conditional_t<std::is_same<Lock,
 std::shared_timed_mutex>::value,
                                                  ^
 }}}

 {{{libcxx}}} has been removing workarounds that existed for older systems,
 and instead marking certain features as unavailable prior to specific OS
 versions.

-- 
Ticket URL: <https://trac.macports.org/ticket/60853#comment:4>
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