Fix for build on Windows

[mdhvg]

• This PR fixes issues in the code that could've lead to undefined behaviours
  • Variable-Length Arrays (VLAs), though supported by GCC and Clang, not recommended for use

float vec[vec_len];
// vec_len being a variable set at runtime.

• Also, the pthread for windows conversion macro from ggml-org/ggml is added to clip.cpp to enable multi-threading support on Windows.

[recurseml[bot]]

😱 Found 1 issue. Time to roll up your sleeves! 😱

[monatis]

Hi @mdhvg, thanks for raising this PR! I'll review it later today and then merge.