signal is determined by the build environment, but the next part seems
to contradict that. Is this just a wording problem (the first sentence
missing a not?) or something more subtle?
Build environment of used eglibc (MIN_KERNEL_SUPPORTED),
but not build environment of your binary.
Plain FreeBSD has a getosreldate here (as I read sbcl source[0]) right?
Maybe that could be added so this test can be kept uniformly?
Yes, but it is the version of running kernel,
but such test needs version of kernel compiled for.