editors/calligra: switch to C++17 for upcoming poppler

This is a fix-it-before-it-breaks commit. Upcoming poppler update
requires C++17 in consumers, while Calligra was fixed on C++14.
There is no upstream release for this.

This is a mess: the upstream repo contains fixes, but they're
mostly committed as "fix warnings" and many commits combine C++17-
related fixes with other bits and pieces. As a result, there's
some direct upstream patches now in files/ and a big glommed-
together one put together from parts of other commits.

PR: 260956