clang and configure checking for equivalent simple type

[Klaus T. Aehlig]

Hi,

> Alternatively, you can turn off only a specific -Werror, e.g.
> 
>   CFLAGS += -Wno-error=unused
> 
> it's ignored by gcc in base while gcc46 wants -Wno-error=unused-value

that's a very good point. Thanks a lot!

Best,
Klaus