[MacPorts] #53601: configure.sdk_version SDK does not always exist

[MacPorts]

#53601: configure.sdk_version SDK does not always exist
---------------------------------+-----------------
 Reporter:  MarcusCalhoun-Lopez  |      Owner:
     Type:  defect               |     Status:  new
 Priority:  Normal               |  Milestone:
Component:  base                 |    Version:
 Keywords:                       |       Port:
---------------------------------+-----------------
 By default, the value of is [https://github.com/macports/macports-
 base/blob/master/src/macports1.0/macports.tcl#L1035 macosx_sdk_version] is
 macosx_version.[[BR]]
 So the default value of [https://github.com/macports/macports-
 base/blob/master/src/port1.0/portconfigure.tcl#L222 configure.sdk_version]
 is also macosx_version.[[BR]]
 Starting with [https://en.wikipedia.org/wiki/Xcode#Xcode_7.0_-_8.x_
 .28since_Free_On-Device_Development.29 Xcode 7.0], the SDK for
 macosx_version is not always provided.[[BR]]
 For example, Xcode 8.2.1 is supported on OS X 10.11 but the only SDK
 provided is 10.12.[[BR]]
 This causes a bit of an issue with Qt (#53597).

 configure.sdk_version is not widely used
 ([https://github.com/macports/macports-
 ports/blob/master/databases/mongodb/Portfile mongodb],
 [https://github.com/macports/macports-
 ports/blob/master/_resources/port1.0/group/qmake5-1.0.tcl qmake5 PG],
 [https://github.com/macports/macports-ports/blob/master/aqua/qt5/Portfile
 qt5], and
 [https://github.com/macports/macports-ports/blob/master/aqua/qt55/Portfile
 qt55]), so any change should cause minimal disruptions.

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Ticket URL: <https://trac.macports.org/ticket/53601>
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