[MacPorts] #55469: snowleopard_fixes portgroup behavior changed, causing some ports to fail to build

[MacPorts]

#55469: snowleopard_fixes portgroup behavior changed, causing some ports to fail to
build
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  Reporter:  ryandesign  |      Owner:  kencu
      Type:  defect      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:
Resolution:              |   Keywords:
      Port:              |
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Comment (by ryandesign):

 Ah yes. That won't work unless the portfile author sets
 `snowleopard_fixes.addheader` before including the portgroup, which would
 look unusual. (The same way that the obsolete-1.0 portgroup requires the
 portfile author to set `replaced_by` before including the portgroup, and
 that looks unusual.)

 The solution is to create an `option_proc` that's called whenever the
 `snowleopard_fixes.addheader` option's value is changed; in that proc, you
 then add or remove flags as needed depending on the value the option is
 being set to.

 `grep` the groups directory for `option_proc` for some examples.

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Ticket URL: <https://trac.macports.org/ticket/55469#comment:7>
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