I think (?) this should be handled in: https://github.com/haikuports/haikuports/blob/be16ca26833ac0180691621f054fe6fa68c96923/dev-libs/boost/patches/boost1.83-1.83.0.patchset#L257C8-L257C62
But seeing this hasn’t hit the beta I guess it didn’t, from the looks of it the binary “b2” is using those flags, not sure now for the rest of the source 