[MacPorts] #39083: Add information about configure.sdkroot and configure.macosx_deployment_target

[MacPorts]

#39083: Add information about configure.sdkroot and
configure.macosx_deployment_target
-------------------------+---------------------
 Reporter:  william@…    |      Owner:  markd@…
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  guide        |    Version:  2.1.3
 Keywords:               |       Port:
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 At present the guide has only a placeholder for the
 configure.macosx_deployment_target and related variables, and no mention
 at all of configure.sdkroot.

 It would be very useful if these (and any others that are missing) had /at
 least/ placeholders (even without any meaningful documentation), so that
 they may be discovered. I recently /guessed/ at the existence of the
 configure.sdkroot variable, and this was exactly what I needed to use to
 get round a problem while testing a port which didn't compile! It would
 have been quicker if I could have seen it somewhere in the guide.

 Also, an explanation of how to discover the list of valid variables
 directly from an installed port tree (for use when the documentation is
 outdated) would also be helpful.

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Ticket URL: <https://trac.macports.org/ticket/39083>
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