Since libuv commit `83efa3dd71` (Reland "macos: use posix_spawn instead
of fork", 2022-03-02, v1.44.0~10), `uv_spawn` on macOS < 10.8
has been observed to cause kernel panics and/or resource exhaustion.
This became particularly noticeable in CMake since commit 5420639a8d
(cmExecuteProcessCommand: Replace cmsysProcess with cmUVProcessChain,
2023-06-01, v3.28.0-rc1~138^2~8). Prefer `fork` over `posix_spawn` in
libuv when targeting macOS < 10.8.
Fixes: #25414Fixes: #25818
Inspired-by: Ken Cunningham <kencu@macports.org>