make: Don't segfault if we can't get a tty name

This happened in VM builds:

  make flags: SHELL=/nix/store/dbxpkswwc7rh6g1iy6dwqklzw39hihb1-bash-4.3-p33/bin/bash
  /nix/store/jm26xg0h3jcrg4bbrwiqx3jpirscdk0p-stdenv/setup: line 658:  5957 Segmentation fault      make ${makefile:+-f $makefile} ${enableParallelBuilding:+-j${NIX_BUILD_CORES} -l${NIX_BUILD_CORES}} $makeFlags "${makeFlagsArray[@]}" $buildFlags "${buildFlagsArray[@]}"