[MacPorts] #69571: Add stpncpy to legacy support

[MacPorts]

#69571: Add stpncpy to legacy support
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  Reporter:  ryandesign      |      Owner:  mascguy
      Type:  enhancement     |     Status:  assigned
  Priority:  Normal          |  Milestone:
 Component:  ports           |    Version:
Resolution:                  |   Keywords:
      Port:  legacy-support  |
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Comment (by ryandesign):

 dash, for example, currently fails to build on Mac OS X 10.6 and earlier
 because of the lack of `stpncpy`; see #69572. And this prevents the
 installation of other ports, like littleutils, that depend on dash.

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Ticket URL: <https://trac.macports.org/ticket/69571#comment:1>
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