After now rereading my message, I want to mention that I don't think
that :UNIX should be reported instead of :NETBSD
There is a bit of confusion here. ECL simply exports several C macros as features. :unix is exported because the compiler reports unix, not because it detects a unix-compatible system. Actually this is a very old definition and it probably meant sco unix back then, as we also have an opposing BSD definition in the same file.
I have change this, though, and #+unix is now reported for all what is now considered unix.