def _get_env_var(for_machine: MachineChoice, is_cross: bool, var_name: str) -> T.Optional[str]:
    """
    Returns the exact env var and the value.
    """
    candidates = PerMachine(
        # The prefixed build version takes priority, but if we are native
        # compiling we fall back on the unprefixed host version. This
        # allows native builds to never need to worry about the 'BUILD_*'
        # ones.
        ([var_name + '_FOR_BUILD'] if is_cross else [var_name]),
        # Always just the unprefixed host versions
        [var_name]
    )[for_machine]
    for var in candidates:
        value = os.environ.get(var)
        if value is not None:
            break
    else:
        formatted = ', '.join([f'{var!r}' for var in candidates])
        mlog.debug(f'None of {formatted} are defined in the environment, not changing global flags.')
        return None
    mlog.debug(f'Using {var!r} from environment with value: {value!r}')
    return value

    Returns the exact env var and the value.
    
mesonbuild/environment.py