New comment by jcgruenhage on void-packages repository

https://github.com/void-linux/void-packages/pull/33356#issuecomment-937727013

Comment:
afaict, the variable should be `CXX_FOR_BUILD`, and the same for the `CXXFLAGS` etc, but even with those it doesn't seem to quite pass the tests. I don't know much about cross compilation together with make, so I'd be happy about input here.