I can't provide you a full solution, but here are the hints: - gcc5 definition: https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/all-packages.nix#L4

I can't provide you a full solution, but here are the hints: - gcc5 definition: https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/all-packages.nix#L4168 https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/... - Old cc wrapper with a custom libc option https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/all-packages.nix#L5188 https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/... - libc in the GCC package itself: https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/compilers/gcc/5/default.nix#L360 https://github.com/NixOS/nixpkgs/blob/master/pkgs/developmen... The whole system is very modular and customizable. But yeah, very hackish and undocumented.