[MacPorts] #52742: buildbot: add support for building ports with variants

[MacPorts]

#52742: buildbot: add support for building ports with variants
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  Reporter:  mojca           |      Owner:  admin@…
      Type:  enhancement     |     Status:  new
  Priority:  Normal          |  Milestone:
 Component:  server/hosting  |    Version:
Resolution:                  |   Keywords:  buildbot
      Port:                  |
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Comment (by mojca):

 The new (trivial) patch for `master.cfg` seems to work for me.

 We need some basic safeguard (regexp?) though to make sure that only valid
 variant names are entered, otherwise people could end up writing port
 names or any other weird things. (I could imagine things equivalent to sql
 injections ending up here. Nothing to do with sql, but I hope you get the
 idea.)

 This will be sufficient to get `gkt3 +quartz` and other binaries on the
 buildbots. The next (independent) step would be to write the logic to
 build that automatically for some ports.

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Ticket URL: <https://trac.macports.org/ticket/52742#comment:7>
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