[fix] update CMakeLists.txt to remove REQUIRED

  # to be uncommented in CI

  # set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror")

  # Prefer clang-18 only when IVAS is built standalone

  # don't override CMAKE_C_COMPILER if it's already set

  if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR AND NOT CMAKE_C_COMPILER)

    # set C compiler

    find_program(cc NAMES clang-18 clang REQUIRED)

    find_program(cc NAMES clang-18 clang)

    if(cc)

    set(CMAKE_C_COMPILER "${cc}" CACHE FILEPATH "")

    else()

    message(WARNING "clang compiler not found - falling back to system default")

    endif()

   endif()

  # CLANG