[MacPorts] #63771: sdrangel: builds fail for macOS 10.11 and earlier; fix, or set known_fail

[MacPorts]

#63771: sdrangel: builds fail for macOS 10.11 and earlier; fix, or set known_fail
---------------------+----------------------
 Reporter:  mascguy  |      Owner:  ra1nb0w
     Type:  defect   |     Status:  assigned
 Priority:  Normal   |  Milestone:
Component:  ports    |    Version:  2.7.1
 Keywords:           |       Port:  sdrangel
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 It looks like this port is failing to build for macOS releases prior to
 10.12. Let's either fix, or set `known_fail` to eliminate wasted buildbot
 time.

 https://ports.macports.org/port/SDRangel/details/

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Ticket URL: <https://trac.macports.org/ticket/63771>
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