Fix amalgamation build: returning (std::)move will otherwise be flagg…

[carlopi]

…ed as not required

Probably there is a better / more general solution to this, since I expect it will pop-up also in other places

[Mytherin]

Is this not related to #2866? Will these changes not break GCC-4.8? Perhaps you could verify this on your fork CI first.

[carlopi]

Is this not related to #2866? Will these changes not break GCC-4.8? Perhaps you could verify this on your fork CI first.

Yes, it is related, I will give it a read to check the reasoning.
I think possibly it should be fine (now, I did run into GCC-4.8 / rasperry pi in a previous version), since we explicitly re-create the unique_ptr<> with the right type, and I think it should work on the CI (but then there was a second hidden issue, this one easier to fix since types are the same).

[Mytherin]

Thanks! Seems like it passes the CI.