[MacPorts] #71942: Make legacy-support library work properly on later OS than build target

[MacPorts]

#71942: Make legacy-support library work properly on later OS than build target
-------------------------+----------------------------
 Reporter:  fhgwright    |      Owner:  fhgwright
     Type:  enhancement  |     Status:  assigned
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:               |       Port:  legacy-support
-------------------------+----------------------------
 In general, most macOS code works properly when run on a later OS than the
 version it was built for, due to general backward compatibility in the OS.
 The converse is not necessarily true, though.

 Since `legacy-support` is providing missing OS functions and wrappers for
 deficient OS functions, it's coupled quite closely to the target OS
 version.  In many cases, running on a later OS may simply make a harmless
 substitution of its own implementation of something to the version
 provided by the OS, but in some cases, its unnecessary replacement version
 may be inferior, especially with a large version mismatch.  So detecting
 such cases at runtime and acting appropriately would be desirable.

 This is likely to be a much more significant issue with the static library
 than with either dynamic library.  There's no good reason to install a
 dynamic library built for a different OS version than the one where it's
 installed (though it might be brought forward with an OS upgrade), but the
 static library might be incorporated into some program which is later used
 under a different OS version.

 In the absence of a general version number in libSystem, this issue
 probably needs to be addressed on a case-by-case basis.

-- 
Ticket URL: <https://trac.macports.org/ticket/71942>
MacPorts <https://www.macports.org/>
Ports system for macOS