Try for adaption to down to avr-gcc 5.2

[ckormanyos]

This is an experimental ticket.

Several important domains still use avr-gcc 5.2, 5.3, or 5.4.

We do, however, find that at least one compiler-intrinsic (i.e., __is_constructible used often in <type_traits>) is unavailable.

1e245c1 works around this phenomenon. It is worth a try to see if the very-detailesmanual re-write of certain std::is_constructible codes might, in fact, with the workaround this for even GCC 5.2.

That manual implementation of __is_constructible was, however, (or so it seems) subsequently removed.

Try to refactor the preprocessor logic here to give it a try on GCC 5.2 using that previous manual implementatoin.