[MacPorts] #58442: gcc9 (and other gccN) fails to install on Mojave, Xcode-10.2.1

[MacPorts]

#58442: gcc9 (and other gccN) fails to install on Mojave, Xcode-10.2.1
-------------------------+--------------------
  Reporter:  mouse07410  |      Owner:  (none)
      Type:  defect      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:
Resolution:              |   Keywords:
      Port:  gcc9        |
-------------------------+--------------------

Comment (by kencu):

 well it was the default at one point:
 {{{
 commit e0aea2323502365a86c06070c936f7dfd2b5dde0
 Author: Chris Jones <jonesc at macports.org>
 Date:   Tue Aug 7 21:00:23 2018 +0100

     cctools: Enforce xcode variant on Xcode 9+ systems
 }}}

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Ticket URL: <https://trac.macports.org/ticket/58442#comment:17>
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