[3.x] Bump to c++17

[lawnjelly]

As discussed, we want to upgrade to c++17 for a few new bits of super useful functionality, especially if constexpr.

I have a later PR for adding some of the compiler requirements we have in 4.x (in SConstruct) but hopefully this should be okay to get started.

Notes

• if constexpr is particularly useful for compile time branching, may be useful in [3.x] Optimize Object::cast_to with ancestral classes when possible #107844 and in a number of cases where we are starting to heavily optimize internal core.

[Ivorforce]

You might want to update some checks, using @Repiteo's C++20 PR as reference: #100749

I'm not sure how much of this transfers, but I expect there should be at least analogous checks in 3.x

[lawnjelly]

Yeah I have a WIP PR already, I'll make it draft : #107851

[Ivorforce]

Good from my side, though we should still get build system feedback :)

[lawnjelly]

On thing that is interesting is that the WIP PR for checking the compiler versions (which corresponds to 4.x with c++17) is flagging one of the compilers as being old, so maybe is a little out of date for c++17 for some features.

But we can discuss changing that on #107844.

I'm happy either going ahead here or waiting till we decide on that, just as long as we can start using if conspexpr soonish. We aren't planning on making builds imminently but it would be nice to sort out the compiler versions.

UPDATE:
Seem to have fixed the compiler version bug so I think we should be good to go.
But I'll wait to get a second opinion from @akien-mga / @Repiteo (as I think Repiteo did a lot of work in checking compiler versions?).

[Repiteo]

This barely scratches the surface of what potentially changed with C++17, but I also don't think this PR was meant to delve into that either. In terms of pure functionality, this is technically all you need… With the caveat that several followup PRs are a certainty.