When building for Apple device platforms, don't expect
CMAKE_SYSTEM_VERSION to be set and don't set
DARWIN_MAJOR_VERSION. In turn, don't erroneously query the latter to
determine whether the -Wl,-search_paths_first linker flag is supported.
It is expected to be available for all Apple SDKs that we now support.

Projects may be relying on the linker flag not being added, so policy
CMP0123 provides backward compatibility for this change.