Fix Nix (#1094)

The current nix expression doesn't build for me on nixos on 23.11 stable
(bunch of various build errors that I can share if useful). This nix
flake builds the project without erroring (e.g. `nix develop -c cargo
build --release`) and also brings in the rust toolchain declaratively
with nix instead of with rustup.

flake.nix

@@ -0,0 +1,59 @@
+{
+  description = "Flake for c2rust";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
+    utils.url = "github:numtide/flake-utils";
+    fenix = {
+      url = "github:nix-community/fenix";
+    };
+    oxalica = {
+      url = "github:oxalica/rust-overlay";
+    };
+  };
+
+  outputs = inputs@{ self, nixpkgs, utils, fenix, oxalica}:
+    utils.lib.eachDefaultSystem (system:
+    let
+        fenixStable = fenix.packages.${system}.fromToolchainFile {
+          file = ./rust-toolchain.toml;
+          sha256 = "sha256-r/8YBFuFa4hpwgE3FnME7nQA2Uc1uqj0eCE1NWmI1u0";
+        };
+        pkgs = import nixpkgs {
+          inherit system;
+          overlays = [
+            (import oxalica)
+          ];
+          config = {
+            allowUnfree = true;
+          };
+        };
+        in {
+          defaultPackage = self.devShell.${system};
+          devShell = pkgs.mkShell.override { } {
+
+            LIBCLANG_PATH = "${pkgs.llvmPackages_14.libclang.lib}/lib";
+            CMAKE_LLVM_DIR = "${pkgs.llvmPackages_14.libllvm.dev}/lib/cmake/llvm";
+            CMAKE_CLANG_DIR = "${pkgs.llvmPackages_14.libclang.dev}/lib/cmake/clang";
+            shellHook = ''
+              export CARGO_TARGET_DIR="$(git rev-parse --show-toplevel)/target_dirs/nix_rustc";
+            '';
+            RUST_SRC_PATH = pkgs.rustPlatform.rustLibSrc;
+            buildInputs =
+              with pkgs; [
+                clangStdenv.cc
+                llvmPackages_14.libclang
+                pkg-config
+                fenix.packages.${system}.rust-analyzer
+                llvmPackages_14.clang
+                cmake
+                llvmPackages_14.llvm
+                llvmPackages_14.libllvm
+                openssl
+                python3
+                zlib
+                (rust-bin.fromRustupToolchainFile ./rust-toolchain.toml)
+              ];
+          };
+    });
+}