[MacPorts] #69162: portaudio: blacklist old Xcode clangs of <10.10

[MacPorts]

#69162: portaudio: blacklist old Xcode clangs of  <10.10
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 Reporter:  barracuda156                   |      Owner:  (none)
     Type:  defect                         |     Status:  new
 Priority:  Normal                         |  Milestone:
Component:  ports                          |    Version:  2.8.1
 Keywords:  lion, mountainlion, mavericks  |       Port:  portaudio
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 https://build.macports.org/builders/ports-10.9_x86_64-builder/builds/260858/steps
 /install-port/logs/stdio

 We need to blacklist old Xcode clangs which do not support stdatomic.

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Ticket URL: <https://trac.macports.org/ticket/69162>
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