[MacPorts] #57539: legacysupport-1.0: Let ports specify what symbols they need

[MacPorts]

#57539: legacysupport-1.0: Let ports specify what symbols they need
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  Reporter:  ryandesign     |      Owner:  (none)
      Type:  defect         |     Status:  new
  Priority:  Normal         |  Milestone:
 Component:  ports          |    Version:
Resolution:                 |   Keywords:
      Port:  legacysupport  |
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Comment (by ryandesign):

 That's great, but if a port only needs e.g. `strnlen`, there is zero
 purpose to linking with the library on 10.7 or later, and the portfile
 author should not have to know that `strnlen` was released in 10.7; the
 portfile author should only need to specify that they need legacy support
 to provide an implementation of `strnlen` on those systems that require
 that.

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Ticket URL: <https://trac.macports.org/ticket/57539#comment:3>
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