[MacPorts] #59113: gcc: cannot find systems headers with Xcode 11

[MacPorts]

#59113: gcc: cannot find systems headers with Xcode 11
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  Reporter:  abyrvalk        |      Owner:  (none)
      Type:  defect          |     Status:  new
  Priority:  Normal          |  Milestone:
 Component:  ports           |    Version:
Resolution:                  |   Keywords:
      Port:  gcc7 gcc8 gcc9  |
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Comment (by jeremyhu):

 1) You don't need to delete Xcode.app.  You can just install both Xcode 11
 and Xcode 10 side by side

 2) You don't need to install
 Command_Line_Tools_macOS_10.14_for_Xcode_10.3.dmg.  I suggest installing
 the latest CLTools (from Xcode 11) as they contain the 10.14 SDK as well.

 --

 Regarding GCC not locating the SDK automatically, you can pass -isysroot
 `xcrun --show-sdk-path` on the command line.

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Ticket URL: <https://trac.macports.org/ticket/59113#comment:18>
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