[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 ryandesign):

 It was said at one point that implementing this type of restriction on
 whether a port can build or not (whether it's based on OS version or other
 criteria) could not be done until we had merged the GSoC '15 libsolv
 branch. Do we still think that's a requirement, or no?

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Ticket URL: <https://trac.macports.org/ticket/15712#comment:34>
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