Bug#915267: [3dprinter-general] Bug#915267: libpolyclipping FTCBFS: does not pass cross flags to cmake

[Gregor Riepl]

I just found out that I previously worked on fixing this, but never finished.
There's a few other issues that make this more complicated than simply cutting
down d/rules to the bare minimum.

I'll prepare an upload once those are sorted out.