Expose the crate's manifest dir to dependent crates

[AllenDang]

What's the problem?

When gdextension-api is used as a build dependency during cross-compilation, dependent crates can't reliably locate platform-specific binding files. The #[cfg] attributes in load_gdextension_header_rs() get evaluated for the HOST platform, not the TARGET, causing builds to fail with the wrong bindings.

The solution

This PR adds a simple build script that exports the crate's manifest directory via cargo:root=.... This lets dependent crates access platform-specific files directly using the standard DEP_GDEXTENSION_API_ROOT environment variable.

What changed?

• Added build.rs that exports CARGO_MANIFEST_DIR as cargo:root
• No changes to existing code or API
• Cargo automatically detects build.rs, no Cargo.toml changes needed

Testing

• ✅ Builds successfully with no changes to existing functionality
• ✅ cargo:root properly exported and available to dependents as DEP_GDEXTENSION_API_ROOT
• ✅ Tested with godot-rust/gdext cross-compilation scenarios

Related

This enables godot-rust/gdext#1361 to fix cross-compilation in a clean way without file system searching.

[Bromeon]

Thanks! Note that this isn't a repo with contributions in the classical sense: it's mostly for artifact storage, partly generated code, and the Git history may be rewritten if that serves the needs of the project. As such I won't merge PRs for now, but I can see if I can integrate your approach in a way so that releases would benefit from this.

We'd need a way to verify that this approach indeed works in downstream godot-rust though. The downstream PR godot-rust/gdext#1361 doesn't mention this environment variable yet... How can we test this?

[AllenDang]

I tested it at my local MacBook with cross compile to windows gnu/msvc/linux/android.

[Bromeon]

But with which code? Your current PR in godot-rust does not mention this environment variable.

We need to be able to reproduce this. Please update godot-rust/gdext#1361 and also keep all unrelated changes out of the diff (like here, there are 2 changes that have nothing to do with the problem).

[AllenDang]

gdext#1361 is updated.