The reason it's not there yet is that I'm unaware of a good way to detect which libc version is installed on a system. Or would it always be libc.so.6?
I suppose libc5 is outdated enough to forget about it and a libc7 is still far away. :)
So the libc6 statement would work for 99% of UNIX users whereas the current situation works for 0% of CLISP users.
Leslie