helper.pl - check that ifdef guard is consistent with function being defined

[minad]

see #373

[sjaeckel]

Was this even at the point of opening a valid issue?

It seems like helper.pl:333 handles this already correctly and #373 was related to bn_deprecated.c which doesn't exist anymore.

[minad]

I think I once had an issue with bn_deprecated. At that point it was a valid issue