nodejs: fix on i686-linux

Fixes this:

  creating  ./config.gypi
  creating  ./config.mk
  building
  make flags: SHELL=/nix/store/fp0nwhj932kscakw1gbnlmmk8pdfv9sq-bash-4.2-p45/bin/bash
  building out/Makefile
  /nix/store/vh2zy8l2797yl3mri35y8jnhc81w9hm1-python-2.7.6/bin/python tools/gyp_node.py -f make
  /bin/sh: which: command not found
  gyp: Call to '(echo | $(echo ${CXX_host:-$(which g++)}) -m32 -E - > /dev/null 2>&1) && echo "-m32" || true' returned exit status 0.
  make: *** [out/Makefile] Error 1

[Bjørn: add build error to commit message.]