[MacPorts] #56241: jack @ 1.9.12_0 fails to upgrade due to using installed headers

[MacPorts]

#56241: jack @ 1.9.12_0 fails to upgrade due to using installed headers
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  Reporter:  ci42    |      Owner:  neverpanic
      Type:  defect  |     Status:  assigned
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:
Resolution:          |   Keywords:
      Port:  jack    |
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Comment (by neverpanic):

 Meh, didn't notice because I was building in trace mode, which prevents
 this problem. Can somebody test wether setting
 `PKG_CONFIG_SYSTEM_INCLUDE_PATH` solves this? If not, I guess we're left
 with the conflicts-build PortGroup, which is not ideal.

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Ticket URL: <https://trac.macports.org/ticket/56241#comment:7>
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