[MacPorts] #54551: ffmpeg @3.3.3_0 fails to build on macOS 10.13

[MacPorts]

#54551: ffmpeg @3.3.3_0 fails to build on macOS 10.13
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  Reporter:  rex4539  |      Owner:
      Type:  defect   |     Status:  new
  Priority:  Normal   |  Milestone:
 Component:  ports    |    Version:  2.4.99
Resolution:           |   Keywords:  highsierra
      Port:  ffmpeg   |
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Comment (by jeremyhu):

 What's the output of `plutil -p
 /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/System/Library/CoreServices/SystemVersion.plist`

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Ticket URL: <https://trac.macports.org/ticket/54551#comment:1>
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