Looking at each usage of DETECT_OS_UNIX, it's more about the POSIX API usage, not the
Unix-like OS, so let's rename it
And for POSIX it's a standard to claim which API present, but for UNIX there is no such thing
Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Acked-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29555>