unistd.h provides isatty. It is provided by 4.3BSD and SVR4, and is
required by POSIX.1-2001. If an old commercial UNIX indeed screw it up,
its libc must lack enough utilities to build LLVM. We should not support
such an ancient system.
isatty() is not supported on the PS4 console OS. We'd like to be able to run some LLVM libraries there, e.g., to support symbolization in compiler-rt on the device. D75278 is a step toward that goal, which I don't think is too unreasonable.
I wonder if other small/embedded devices would have similar restrictions e.g. a watch. Add @aprantl for an opinion.
I'd rather you did not do this.