[poppler] Increasing the C++ standard to C++17

[Albert Astals Cid]

https://gitlab.freedesktop.org/poppler/poppler/-/merge_requests/941

Anyone against it?

It would allow us to use std::optional which would allow us to change things like

std::unique_ptr<GooString> readMetadata();

to 

std::optional<std::string> readMetadata();

where you can differentiate between "no metadata" and "empty metadata" without having to use a pointer.

Seems better API to me.

All the things in the gitlab CI built fine and I don't think the mac/windows builders would be a problem to get the increased C++ standard.

Cheers,
  Albert