cmake: Adjust macOS SDK detection

[jcm93]

Description

Adjust the macOS CMake initialization logic to use xcrun to derive our SDK version information, rather than relying on the presence of ${CMAKE_OSX_SYSROOT}.

Motivation and Context

CMake 4.0.0 (currently in the release candidate phase) breaks OBS configuration/generation on macOS. This is because CMake 4.0.0 no longer defines CMAKE_OSX_SYSROOT by default for macOS builds, which we currently rely on to make sure the user has an SDK supported by OBS.

This CMake change was apparently made because specifying CMAKE_OSX_SYSROOT seems to translate under the hood to passing -isysroot to the compiler, which apparently had some undesired impacts in some situations (further context here and here). Letting the compiler wrapper figure out its sysroot on its own seems to now be the preferred pattern.

As we would still like to check what SDK version the user has installed, we can just derive it ourselves using xcrun to populate a variable with the user's environment's currently configured SDK path. This way we can make sure that the user has an SDK version we support, while still letting CMake not define a sysroot.

How Has This Been Tested?

Installed CMake 4.0.0 rc1 and configured/generated/built OBS successfully; also made sure configuration/generation/building proceeded successfully on CMake 3.31.4.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.