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

[Gregor Riepl]

> libpolyclipping fails to cross build from source, because it does not
> pass cross flags to cmake. The easiest way of fixing that is using
> dh_auto_configure. The attached patch does that and simplifies
> debian/rules a little using the --sourcedirectory flag. Please consider
> applying the patch.

If --sourcedirectory is indeed enough to make dh detect that cmake should be
used, I'll gladly apply the patch.
Thanks!