[CMAKE] Allow threads to be not found for cross compilation

[tqchen]

This PR updates the threads linkage to be optional so the library can be build for cross-compilation targets where threads may not be available.

[tqchen]

Needed for hexagon build

[gemini-code-assist]

Summary of Changes

Hello @tqchen, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request modifies the CMake build configuration to make the linkage of thread libraries optional. This change is primarily aimed at improving support for cross-compilation environments, enabling the library to be built successfully even when thread-related dependencies are not present or are handled differently in the target system. By removing the strict requirement for Threads, the build process gains greater flexibility.

Highlights

• Optional Threads Linkage: The Threads package is no longer a mandatory dependency during CMake configuration, allowing builds to proceed even if thread libraries are not found.
• Enhanced Cross-Compilation Support: The build system now gracefully handles scenarios where thread libraries are unavailable, specifically improving support for cross-compilation targets.
• Conditional Library Linking: The tvm_ffi_shared and tvm_ffi_static targets are now conditionally linked against Threads::Threads only if the Threads package is successfully located by CMake.

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[gemini-code-assist]

Code Review

This pull request aims to make thread support optional for cross-compilation. The approach of making find_package(Threads) optional can lead to confusing link-time errors if thread detection fails on platforms that require it. I've suggested a more robust approach using a dedicated CMake option (TVM_FFI_WITH_THREADS) to explicitly control thread support. This would involve conditionally compiling thread-dependent code, making the build system more predictable and easier to debug for all users.

[junrushao]

@tqchen Could you adopt Gemini’s recommendation in a follow up PR?