[MacPorts] #58260: libgcc8 build fails due to "Atomic" error on Mojave

[MacPorts]

#58260: libgcc8 build fails due to "Atomic" error on Mojave
---------------------------+--------------------
  Reporter:  physicsbeany  |      Owner:  (none)
      Type:  defect        |     Status:  new
  Priority:  Normal        |  Milestone:
 Component:  ports         |    Version:  2.5.4
Resolution:                |   Keywords:  mojave
      Port:  libgcc8       |
---------------------------+--------------------

Comment (by cjones051073):

 It seems this is being found in a number of places. e.g.

 https://apple.stackexchange.com/questions/355049/compilation-error-with-
 mojave-error-atomic-does-not-name-a-type

 https://discourse.brew.sh/t/atomic-does-not-name-a-type-gcc8-should-be-
 updated-patched/4516

 Its looking to me like something new to the recent macOS 10.14.4 and Xcode
 10.2 update.

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Ticket URL: <https://trac.macports.org/ticket/58260#comment:8>
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