Is this going to be τ vs π all over again?

I recently did a bit of programming exploration using tau exclusively instead of pi (and `sintau` instead of the new `sinpi` function etc.)

In almost every aspect this was far simpler, but there was the curious case of the constant `M_2_SQRTPI = 2 / sqrt(π)`. Even after looking up what weird formula that constant is used in, it wasn't at all clear to me where would be the most sensible place to put the constant.