[MacPorts] #58260: gcc{6, 7, 8, 9} : Build failures due to "Atomic" error with macOS 10.4.4 and Xcode 10.2

[MacPorts]

#58260: gcc{6,7,8,9} : Build failures due to "Atomic" error with macOS 10.4.4 and
Xcode 10.2
-------------------------------------+-------------------------------------
  Reporter:  physicsbeany            |      Owner:  Nullstelllensatz
                                     |  <50169837+Nullstelllensatz@…>
      Type:  defect                  |     Status:  closed
  Priority:  Normal                  |  Milestone:
 Component:  ports                   |    Version:  2.5.4
Resolution:  fixed                   |   Keywords:  mojave
      Port:  gcc9 gcc8 gcc7 gcc6     |
  libgcc-devel libgcc8 libgcc7       |
  libgcc6                            |
-------------------------------------+-------------------------------------
Changes (by jmon12):

 * Attachment "Portfile-gcc6.diff" added.

 Condition for applying the _Atomic patch against the SDK version instead
 of Xcode version.

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