Changed C++ standard to C++17

[emjotde]

Hi all,
with the current sync we allow for C++17 now. Have fun.

[emjotde]

And the first piece of code that actually uses that is here:

marian-dev/src/layers/guided_alignment.h

Line 50 in b8bf086

auto [indices, values] = guidedAlignmentToSparse(batch);

[kpu]

intgemm got much easier to refactor.

[emjotde]

Gimme those sweet column-wise scalars :)