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

[MacPorts]

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

Comment (by ogourgue):

 Is there something specific to do when downgrading from Xcode 11 to 10.3?
 Or is it simply:

 1) deleting Xcode.app
 2) downloading and installing Xcode 10.3.xip and
 Command_Line_Tools_macOS_10.14_for_Xcode_10.3.dmg from Apple Developper
 website?

 In particular, is there something to do about the ports successfully
 upgraded with Xcode 11 before one port failed to upgrade (openblas in my
 case)?

 Thanks for your help!

-- 
Ticket URL: <https://trac.macports.org/ticket/59113#comment:17>
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