Bridge Python3_INCLUDE_DIRS when only the unversioned Python module was found

[take-cheeze]

Summary

find_package(Python 3 ...) and find_package(Python3 ...) are separate CMake modules with separate hint/output variable namespaces. nanobind's own CMake config reads the versioned, plural Python3_INCLUDE_DIRS variable specifically — it's never populated by the unversioned find_package(Python ...) call this file uses for ONNX_BUILD_PYTHON.

On a normal host build this goes unnoticed, because something else in the dependency graph (onnx's own CMakeLists.txt) has usually already called find_package(Python3) and populated it first. Cross-compiling can reach this find_package(Python) call with nothing else having populated the Python3_* variables yet — nanobind_add_module then fails with Python.h file not found immediately afterward, despite Development.Module having just been found successfully one line above.

Found while cross-compiling onnxsim (github.com/onnxsim/onnxsim) for Pyodide/wasm32-emscripten — see that repo's docs/wasm_pyodide.md for the full context. Its own build script currently has to work around this by passing both Python3_* and unversioned Python_* CMake hints from the calling side; this fix bridges the gap at the source instead.

The fix

Minimal and guarded: after the existing find_package(Python 3 ...) call, set Python3_INCLUDE_DIRS from Python_INCLUDE_DIRS only if the former isn't already defined. No behavior change to the find_package call itself, and it's a no-op on any build where Python3_INCLUDE_DIRS is already populated (i.e. every normal host build) — it only kicks in for the specific cross-compile ordering gap described above.

Test plan

• Confirmed this is the exact root cause via direct investigation while cross-compiling onnxsim for wasm32-emscripten (reproduced the Python.h file not found failure, confirmed it disappears once Python3_INCLUDE_DIRS is populated)
• Not separately re-verified in this PR against a fresh wasm32 rebuild (the fix mirrors a workaround already validated end-to-end in the downstream onnxsim investigation) — happy to re-verify if requested

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