[chaosfactor]

This does not generalize to arbitrary purposes. If you just want to detect that the user has run more jobs than your build system can possibly benefit from and print a warning, there is no reason to start messing with the jobserver. This is not the Ockham solution :)

[JoshTriplett]

Why print such a warning? For most projects, make -j65536 will be effectively equivalent to make -j, both of which are perfectly fine. Suppose you're on a system with more CPUs than you have build rules in your build system; you shouldn't print a warning just because someone does make -j$(nproc).