Added wayyyy back in 2676cf9525, this NIX_CFLAGS_COMPILE usage is now unneeded. It *necessarily* had to be unneeded, as using kernelModuleMakeFlags and the kernel's out-of-tree build tooling forces the unwrapped `cc` to be used. See here: https://github.com/NixOS/nixpkgs/blame/f86e9fe549f551d6bb738a1d01efa7d61b2da3e7/pkgs/os-specific/linux/kernel/common-flags.nix#L10-L11
2676cf9525