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

[MacPorts]

#52742: buildbot: add support for building ports with variants
----------------------------+------------------------
  Reporter:  mojca          |      Owner:  neverpanic
      Type:  enhancement    |     Status:  assigned
  Priority:  Normal         |  Milestone:
 Component:  buildbot/mpbb  |    Version:
Resolution:                 |   Keywords:  buildbot
      Port:                 |
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Comment (by ryandesign):

 In [changeset:"32fd667ad3d4cb8fe9be7bf2dc5058cc73d8e530/macports-ports"
 32fd667ad3d4cb8fe9be7bf2dc5058cc73d8e530/macports-ports]:
 {{{
 #!ConfigurableCommitTicketReference repository="macports-ports"
 revision="32fd667ad3d4cb8fe9be7bf2dc5058cc73d8e530"
 glib2-devel: Default to +quartz when -x11 is given

 When building pango -x11 in a clean prefix, glib2 will fail to install,
 because the -x11 variant is being passed down to glib2, but glib2
 requires either +quartz or +x11, and +quartz was not enabled by default
 when x11 was disabled.

 This caused problems when implementing variant support for the buildbot
 in https://github.com/macports/mpbb/pull/5 and initially caused me to
 think that I would have to revert
 https://github.com/macports/mpbb/commit/f6e46815a5033358212ce383956fd567eb51e53c
 which was added due to https://github.com/macports/mpbb/pull/4 and
 https://lists.macports.org/pipermail/macports-dev/2017-June/035978.html.

 This solution should instead work without the revert and still allow the
 buildbot to build both wine and +quartz-x11 ports.

 See: https://trac.macports.org/ticket/52742
 }}}

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Ticket URL: <https://trac.macports.org/ticket/52742#comment:20>
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