That is basically "what it means" if an interface is non-MT: you can call this no-problem if you know you're singlethreaded, and if you're not, find your own way to serialize (meaning: have your own locking prinitive you acquire/release where you make calls to these functions).
One could "dream of" a func that tells libc "acquire/drop this mutex of mine around get/set/putenv calls" but that'd simply move the problem - because the nifty "frameworks" would do that (independently of each other, we're sovereign and entitled frameworks around here) and race each other's state nonetheless.
