fix(rust): fix SIMDe configuration on aarch64 platforms when not installed yet

[yutannihilation]

(In order to fix build on aarch64 platfroms, #62 is still needed in addition to this change)

I was wondering why the FastPFOR-rs repository tests macOS runner without installing SIMDe; FastPFOR fetches and build SIMDe automatically. However, it seems the include path and define are not configured correctly when with cxx_build::bridge(). So, this pull request attempts to fix it by manually doing the equivalent to this part of cmake_modules/simde.cmake.

target_include_directories(simde INTERFACE "${simde_SOURCE_DIR}")

# Enables native aliases. Not ideal but makes it easier to convert old code.
target_compile_definitions(simde INTERFACE SIMDE_ENABLE_NATIVE_ALIASES)

[Copilot]

Pull request overview

This PR adjusts the Rust build script so the CXX bridge compilation on aarch64 mirrors FastPFOR’s SIMDe configuration when SIMDe is fetched by CMake (instead of preinstalled), and updates the README to reflect that SIMDe is typically not a manual dependency on Apple Silicon.

Changes:

• Update build.rs to reuse CMake-fetched SIMDe headers and add the SIMDE_ENABLE_NATIVE_ALIASES define for the CXX bridge on aarch64.
• Add a build-time warning when SIMDe headers can’t be found in the CMake output.
• Update README platform instructions to make SIMDe installation optional and describe fallback options.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
build.rs	Detect aarch64 and try to add SIMDe include path + compile define to the CXX bridge compilation.
README.md	Update build prerequisites to make SIMDe optional and clarify macOS/Apple Silicon expectations.

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[codecov]

Codecov Report

❌ Patch coverage is 44.44444% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
build.rs	44.44%	10 Missing ⚠️

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[nyurik]

thanks!