[MacPorts] #58426: Portfile instructions: unclear about global variable ${portbuildpath}

[MacPorts]

#58426: Portfile instructions: unclear about global variable ${portbuildpath}
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  Reporter:  frankliuao   |      Owner:  (none)
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  guide        |    Version:
Resolution:               |   Keywords:
      Port:               |
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Comment (by jmroot):

 It's one of those variables like `prefix` that is handed down from above.
 Its value is not based on anything that a Portfile can control and it
 doesn't have a fixed default. The way it is constructed is also difficult
 to explain concisely. Have a look at the `macports::getportbuildpath`
 procedure if you're interested.

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Ticket URL: <https://trac.macports.org/ticket/58426#comment:3>
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