Noob question - Trying to build Clang from LLVM Source, getting linker errors to GLIBC library symbols

Clang Frontend Building Clang
1

Here is my super simple flake.nix:

{
  description = "A template for Nix based C++ project setup.";

  inputs = {
    # nixpkgs.url = "github:NixOS/nixpkgs/unstable";
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
  };

  outputs = { self, nixpkgs }: {

    defaultPackage.x86_64-linux =
      # Notice the reference to nixpkgs here.
      with import nixpkgs { system = "x86_64-linux"; };
      stdenv.mkDerivation {
        name = "llvm-env";
        src = self;

        # where to find libgcc
        NIX_LDFLAGS="-L${gccForLibs}/lib/gcc/${targetPlatform.config}/${gccForLibs.version}";
        # teach clang about C startup file locations
        CFLAGS="-B${gccForLibs}/lib/gcc/${targetPlatform.config}/${gccForLibs.version} -B ${stdenv.cc.libc}/lib";

        cmakeFlags = [
          "-DC_INCLUDE_DIRS=${stdenv.cc.libc.dev}/include"
          "-GNinja"
          # Debug for debug builds
          "-DCMAKE_BUILD_TYPE=Release"
          # inst will be our installation prefix
          "-DCMAKE_INSTALL_PREFIX=../inst"
          "-DLLVM_INSTALL_TOOLCHAIN_ONLY=ON"
          # change this to enable the projects you need
          "-DLLVM_ENABLE_PROJECTS=clang"
          # enable libcxx* to come into play at runtimes
          "-DLLVM_ENABLE_RUNTIMES=libcxx;libcxxabi;libunwind"
          # this makes llvm only to produce code for the current platform, this saves CPU time, change it to what you need
          "-DLLVM_TARGETS_TO_BUILD=host"
        ];
      };
  };
}

I run

nix develop --extra-experimental-features "nix-command flakes"

Then follow the steps in this guide to build clang: https://nixos.wiki/wiki/LLVM

However I keep running into “undefined reference to `__isoc23_strtoull’”. I checked and this comes from the libc.so library…

My C_INCLUDE_DIRS are /nix/store/ij144ma6vs8acil8r9hgr8xkb1dp9azg-glibc-2.39-5-dev/include
The library libc.so is located at /nix/store/35pq4hr29c3sl79lgfwgsvd9nwzyp4am-glibc-2.39-5/lib/libc.so
I tried updating LIBRARY_PATH / LD_LIBRARY_PATH to that location but it caused more build issues.

Any ideas on how to fix this?