[MacPorts] #71682: boost187: build fails for 10.10 and earlier: missing TARGET_OS_IOS

[MacPorts]

#71682: boost187: build fails for 10.10 and earlier: missing TARGET_OS_IOS
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 Reporter:  mascguy  |      Owner:  mascguy
     Type:  defect   |     Status:  assigned
 Priority:  Normal   |  Milestone:
Component:  ports    |    Version:  2.10.5
 Keywords:           |       Port:  boost187
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 Use of `legacysupport` is probably the way to go, as there are also
 several missing identifiers for 10.7 and earlier.

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Ticket URL: <https://trac.macports.org/ticket/71682>
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