If we really need to cater to the practice of building for an older 
system than the build host (which to my mind is asking for trouble), 
then instead of having Gnulib try to work around the bugs in 
copy_file_range in Linux kernels before 5.3, it should simply not use 
copy_file_range in these older kernels.

In other words, something like the attached patch should be more 
reliable than the other band-aids proposed.