[Nix-dev] second revision of NUM_CORES patch series

[Marc Weber]

Nix:

  pass NIX_MAX_PARALLELIZATION and NIX_TARGET_LOAD to the builder.
  This is currently only used the default builder running make.
  See patch to NixPkgs

NixOS:

  Add comment about how to set NIX_MAX_PARALLELIZATION and NIX_TARGET_LOAD

NixPKGS:
  
  - setup.sh: Refactor running make because -j and -l with opt-out possibility are added
  - pass -j and -l to make which default to 1 unless you pass NIX_* vars to nix-daemon