fix: resolve build failures on Linux with Clang 18 (const qualifier + OpenMP linker)

[parthalon025]

Summary

Two related build failures on Linux systems with Clang 18.

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Fix 1 — const qualifier on y_col (ggml-bitnet-mad.cpp:811)

Problem: ggml_vec_dot_i2_i8_s_Nx1 receives vy as const void *. Pointer arithmetic on the cast value yields const int8_t *, but the local variable y_col was declared as int8_t *, silently dropping the const qualifier. Clang 18 rejects this as an error; GCC and older Clang accepted it silently.

Fix: Declare y_col as const int8_t * to correctly propagate the const qualifier. Zero behavioral change.

error: cannot initialize a variable of type 'int8_t *' with an rvalue of type 'const int8_t *'
    int8_t * y_col = y + col * by;

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Fix 2 — Auto-detect versioned Clang on Linux (setup_env.py)

Problem: On Linux systems where both a distro-packaged clang-18 (apt/llvm.org) and an alternative clang (e.g. Homebrew) are installed, the bare clang alias may resolve to a binary that links against a different libomp than the one installed by the OS. This causes a linker error at build time:

undefined reference to `__kmpc_fork_call@VERSION`

Root cause: Each Clang distribution ships its own libomp. Distro-packaged versioned binaries (clang-18, clang-17, ...) are guaranteed to pair with the correct libomp.

Fix: Add _find_clang() which probes for versioned Clang/Clang++ binaries (18 → 17 → 16 → 15) on Linux before falling back to bare clang/clang++. Non-Linux platforms (macOS, Windows) are unaffected.

Tested on: Ubuntu 24.04 with apt install clang-18 libomp-18-dev, x86_64, model Falcon3-10B-Instruct-1.58bit.

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Checklist

• Both changes are backward-compatible
• No new dependencies added
• Tested end-to-end: setup_env.py --hf-repo tiiuae/Falcon3-10B-Instruct-1.58bit builds and runs inference successfully after both fixes

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