Fix compilation errors with libstdc++v3 for AVR target and allow --enable-libstdcxx

[Jonathan Wakely]

On 15/12/16 21:41 -0300, Felipe Magno de Almeida wrote:
>Good point. Do you want me to update the patch with __men or go your way
>with the wrapper?

I think my wrapper's too ugly :-)

So please follow the same approach as for tm_mon (pass __mem to the
function, the copy it to tm_mon if there was no error). That adds a
write and a branch for every extracted field, which I hope shouldn't
have too much impact.