[MacPorts] #60167: lilypond 2.20.0: +docs variant will not build

[MacPorts]

#60167: lilypond 2.20.0: +docs variant will not build
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  Reporter:  JunTamura1959  |      Owner:  nerdling
      Type:  defect         |     Status:  assigned
  Priority:  Normal         |  Milestone:
 Component:  ports          |    Version:  2.6.2
Resolution:                 |   Keywords:
      Port:  lilypond       |
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Comment (by kencu):

 good detective work!

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Ticket URL: <https://trac.macports.org/ticket/60167#comment:10>
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