macOS: Restore support for LLVM/Clang without explicit CMAKE_OSX_SYSROOT

Brad Kingrequested to merge
brad.king/cmake:clang-macos-sdk into master

In !9977 (merged) we broke support for using upstream LLVM/Clang to build for macOS because the compiler has no default sysroot. Handle empty CMAKE_OSX_SYSROOT with LLVM/Clang by falling back to the macOS SDK reported by xcrun --show-sdk-path .

Fixes: #26863 (closed)
Backport: release

Merge request reports