Fix OpenMP detection on macOS with Homebrew libomp

[pit-crew]

CMake's find_package(OpenMP) fails on macOS with Apple Clang even when libomp is installed via Homebrew, because Apple Clang doesn't ship OpenMP discovery hints.

The fix queries brew --prefix libomp and sets the necessary compiler/linker flags before calling find_package(OpenMP). This is a no-op on non-Apple platforms.

Branch: https://github.com/pit-crew/Quant-Kernel-fork/tree/add-homebrew-libom-support