[fix] don't force CMAKE_C_COMPILER unconditionally in CMakeLists.txt

  # 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)

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

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

  endif()

  # CLANG

  if(CLANG)