[unix] configure: reduce clutter due to spurious warnings
[unix] configure: reduce clutter due to spurious warnings

Clang 'only' warns about unknown  -W.. flags. While OK on its own,
it's better to not add the flag nevertheless, so force fail with -Werror

OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm