[MacPorts] #15712: Add versions to platforms

[MacPorts]

#15712: Add versions to platforms
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  Reporter:  raimue       |      Owner:  larryv
      Type:  enhancement  |     Status:  assigned
  Priority:  Normal       |  Milestone:  MacPorts 2.6.0
 Component:  base         |    Version:
Resolution:               |   Keywords:
      Port:               |
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Comment (by mojca):

 One potential advantage would be that, say, Qt5 requires nearly the latest
 OS. If one port includes the PortGroup to request Qt5 (unless special
 measures are taken and an older version is provided, but forget about that
 for a moment), the PortGroup could blacklist some darwin versions and the
 dependent port would be blacklisted automatically. Similarly, if a port
 asks for wxWidgets 3.2, the PortGroup could add
 {{{
 platforms-blacklist-append {darwin < 11}
 }}}
 and the port would automatically cease to exist on 10.6 and lower.

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Ticket URL: <https://trac.macports.org/ticket/15712#comment:19>
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