[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):

 `cctools +xcode` is as close as we can get to not having `cctools`
 installed at all. It defers all the toolchain to `xcode`. (same as `ld64
 +xcode`).

 There may be some way that is not ideal, but I can't think of what it is
 now.

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Ticket URL: <https://trac.macports.org/ticket/58442#comment:15>
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