[MacPorts] #61647: mpv build fails on Mojave

[MacPorts]

#61647: mpv build fails on Mojave
--------------------------+--------------------
 Reporter:  ShadSterling  |      Owner:  (none)
     Type:  defect        |     Status:  new
 Priority:  Normal        |  Milestone:
Component:  ports         |    Version:  2.6.4
 Keywords:                |       Port:  mpv
--------------------------+--------------------
 mpv @0.33.0 on macOS 10.14.6 (18G6042) with Xcode 11.3.1 (11C504)

 {{{
 :info:build <unknown>:0: error: unable to load standard library for target
 'x86_64-apple-macosx10.14'
 }}}

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Ticket URL: <https://trac.macports.org/ticket/61647>
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