[MacPorts] #56669: ldns 1.7.0 ./configure fails to find the SDK on MacOS X <= 10.6

[MacPorts]

#56669: ldns 1.7.0 ./configure fails to find the SDK on MacOS X <= 10.6
-------------------------+-------------------------------
  Reporter:  grumpybozo  |      Owner:  (none)
      Type:  defect      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:  2.5.2
Resolution:              |   Keywords:  haspatch upstream
      Port:  ldns        |
-------------------------+-------------------------------

Comment (by ryandesign):

 Replying to [ticket:56669 grumpybozo]:
 > The root cause is reliance on "xcrun --show-sdk-version" to identify the
 proper SDK version. The "--show-sdk-version" option did not exist in the
 xcrun utility until MacOS X 10.7 (XCode 4.)

 `xcrun --show-sdk-version` does not work before OS X 10.8 / Xcode 5.

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Ticket URL: <https://trac.macports.org/ticket/56669#comment:2>
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