You discovered a bug in the engine, and you are sure that it is not a problem of your code? Just post it in here. Please read the bug posting guidelines first.
The define is only used with IRR_FASTMATH or whatever this mode is called, and it has several pitfalls. If nt enabled, there's a proper inline function in the namespace. I guess burningswater will cope with this issue soon. Maybe submit a bug ticket for it.