New comment by tornaria on void-packages repository

https://github.com/void-linux/void-packages/pull/30036#issuecomment-815413784

Comment:
> I think assigning to `CXXFLAGS` in `pre_build` overrides build flags, including `-O2`. The behavior should be more consistent if you do `CXXFLAGS+=...` in `pre_build`.

Indeed, `-O2` is making the difference. I found a way to round which makes tests robust.

If you think it's ok to merge, I'll squash.