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

[MacPorts]

#57539: legacysupport-1.0: Let ports specify what symbols they need
------------------------+---------------------------
 Reporter:  ryandesign  |      Owner:  (none)
     Type:  defect      |     Status:  new
 Priority:  Normal      |  Milestone:
Component:  ports       |    Version:
 Keywords:              |       Port:  legacysupport
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 The legacysupport-1.0 portgroup currently defaults to doing its thing
 (adding the `port:legacy-support` dependency and manipulating the flags)
 on OS X 10.11 and earlier. Presumably this is for ports that require the
 `clock_gettime` functionality which was new in macOS 10.12. But many ports
 don't need that; many ports are only using this portgroup to add `strnlen`
 or `strndup` for Mac OS X 10.6 or older. Linking with the legacy library
 is unnecessary Mac OS X 10.7 or later in those cases, but the portgroup
 will still do it because the portgroup doesn't know not to.

 The portgroup provides the `legacysupport.newest_darwin_requires_legacy`
 option, which defaults to `15` and which ports can override. But this
 requires each Portfile author to know which versions of macOS introduced
 support for which symbols. It would be better if Portfile authors didn't
 have to know that.

 How about if the portgroup allowed the Portfile author to specify which
 symbols they require? Knowledge about which versions of macOS first
 introduced support for which symbols can be added to the portgroup, so
 that it can set `legacysupport.newest_darwin_requires_legacy` to the
 correct value automatically.

-- 
Ticket URL: <https://trac.macports.org/ticket/57539>
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