Add ability to force overwrite LIBCLANG_PATH for build from nix

[skibon02]

Bug description

I was getting myself familiar with nix, and faced some issues while compiling esp-idf rust project.

Since using rustup is not a good thing in nix (everything should be declarative and separate tools managing packets or versions are generally discouraged), I've created a flake locally in my project dir, which would prepare clean environment with nightly toolchain ready (cargo and rustc). Therefore I don't have rustup and espup in my build environment.

I've created empty test project and minimal nix configuration, and I am getting compilation error:

  thread 'main' (1217628) panicked at /home/user/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/bindgen-0.71.1/lib.rs:604:27:
  Unable to find libclang: "couldn't find any valid shared libraries matching: ['libclang.so', 'libclang-*.so', 'libclang.so.*', 'libclang-*.so.*'], set the `LIBCLANG_PATH` environment variable to a path where one of these files can be found (invalid: [])"

After a few hours trying to fix this, trying different configurations and setting env variables, I came up with a solution, but it requires changes in esp-idf-sys crate:

1. clone esp-idf-sys locally and make changes to function setup_clang_env in build::common:

pub fn setup_clang_env(path: Option<&Path>) -> Result<()> {
    // If the user already set LIBCLANG_PATH, respect it and skip auto-detection
    if std::env::var_os("LIBCLANG_PATH").is_some() {
        return Ok(());
    }

    // ...

2. modify flake.nix

From

    devShells.x86_64-linux.default =
      let pkgs = nixpkgs.legacyPackages.x86_64-linux;
      in pkgs.mkShell {
        buildInputs = [ pkgs.rustup pkgs.ldproxy ];
        shellHook = "rustup show";
      };

to

    devShells.x86_64-linux.default =
      let pkgs = nixpkgs.legacyPackages.x86_64-linux;
      in pkgs.mkShell {
        buildInputs = [ pkgs.rustup pkgs.ldproxy ];
        packages = [
            pkgs.llvmPackages.libclang
        ];
        env.LIBCLANG_PATH = "${pkgs.llvmPackages.libclang.lib}/lib";
      };

And after this project builds successfully.

• Would you like to work on a fix? y

To Reproduce

1. Clone https://github.com/skibon02/esp-nix-issue
2. Install nix package manager
3. In project directory run nix develop and cargo build -r

Expected behavior

Build without errors

Environment

• Crate (esp-idf-sys) version: 0.37.1
• ESP-IDF branch or tag: 5.4.3
• Target device (MCU): esp32c6
• OS: Linux Mint 22.2, Nix package manager with environment from flake

[skibon02]

I am not sure if this fix universal and generally good, but it worked for me

The problem is that without this fix, LIBCLANG_PATH got overwritten by internal algorithms of auto-detection and I did not find a way to correctly override it

[skibon02]

Cloning esp-idf locally and specifying it in package.metadata.esp-idf-sys with esp_idf_tools_install_dir and idf_path also had problems while building