macOS: Avoid relying on sw_vers when cross-compiling from another OS

We have a few code paths needed only to support builds hosted on
macOS 10.4.  Update the conditions to work when cross-compiling
to macOS from another platform.

This backports commit eb8facd396 (macOS: Avoid relying on sw_vers
when cross-compiling from another OS, 2025-04-28, v4.1.0-rc1~260^2)
to CMake 4.0.

Fixes: #26814
2 files changed