Sure, and I don't disagree. I was just using an old OS to make a point about corner cases; it would be just as applicable if I had a modern OS that for whatever reason lacked strdup(), or your personal favorite "but everyone has this!" function. You're not going to be able to cover all bases all the time, and I'm sure that there are plenty of code authors who aren't interested in formally supporting anything outside of the most common operating systems. If their autotools-based projects work on my other OS that's great, but it isn't the fault of autotools if the project isn't coded with my OS in mind.
Normally in modern software, "has it or not" is controlled by some pre-processor variable you can check. The problem comes in when you have under-conformant systems that claim conformance with POSiX 1-20xx, but that lack that one interface mandated by it (and one that's not controlled by some other thing... posix is super complex, for good and for ill). And you also have the edge case of "newly defined in C11" say, and the base compiler doesn't claim C11 conformance, but this function is none-the-less available. It's really really hard to know if it's there or not w/o testing for it. That even goes for "it's a linux box" since musl vs glibc has variations that you won't know about until you check.
So it doesn't have to be something that should be as ubiquitous as strdup to run into issues.
Warner