[MacPorts] #49064: yaml-cpp: error: 'unistd.h' file not found

[MacPorts]

#49064: yaml-cpp: error: 'unistd.h' file not found
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  Reporter:  mikemadden42@…  |      Owner:  macports-tickets@…
      Type:  defect          |     Status:  new
  Priority:  Normal          |  Milestone:
 Component:  ports           |    Version:  2.3.4
Resolution:                  |   Keywords:
      Port:  yaml-cpp        |
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Comment (by mikemadden42@…):

 Please close this ticket.  Running the following before installing the
 port fixed the issue:
 {{{
 xcode-select --install
 }}}

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Ticket URL: <https://trac.macports.org/ticket/49064#comment:2>
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