[MacPorts] #39412: Please document active_variant portgroup

[MacPorts]

#39412: Please document active_variant portgroup
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 Reporter:  thibaut@…    |      Owner:  markd@…
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  guide        |    Version:  2.1.3
 Keywords:               |       Port:
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 Hi,

 The ability to depend on a specific variant of a port has often been
 sought for. It is now provided by means of the active_variant port group,
 but this is not documented in the guide.

 It would be good to have:
 1. a mention of the fact in Sect. 5.3, (dependencies);
 2. complete documentation of the portgroup in 5.9 (portgroups).

 Kind regards, Thibaut.

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Ticket URL: <https://trac.macports.org/ticket/39412>
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