[CMAKE] Allow explicit turning threads off

[tqchen]

No description provided.

[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 enhances the CMake build system by introducing a dedicated option to manage the dependency on the Threads library. This provides greater flexibility for users to explicitly enable or disable threading support during the build process, which can be beneficial for specific deployment scenarios or environments where threading is not required or causes conflicts.

Highlights

• New CMake Option for Threading Control: Introduced the TVM_FFI_USE_THREADS CMake option, defaulting to ON, to explicitly control whether the tvm_ffi library links against the Threads library.
• Conditional Thread Linking: The tvm_ffi target now conditionally links Threads::Threads based on the state of the new TVM_FFI_USE_THREADS option.
• Required Thread Package: When TVM_FFI_USE_THREADS is enabled, find_package(Threads REQUIRED) is used, making the Threads package a mandatory dependency for the build to succeed.

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[gemini-code-assist]

Code Review

This pull request introduces a new CMake option, TVM_FFI_USE_THREADS, to explicitly control linking against the threads library. This change improves the build system's configurability by making the dependency on threads optional and explicit. When the option is enabled, the build will now correctly fail if the threads library is not found, which is a clearer behavior than the previous implementation. The change is well-implemented, though I've noted a minor typo in the new option's description.

[junrushao]

indent this line