[Patch] Fix ONE_DIRECTION undef warnings.

[Roland McGrath]

> I agree that fixing this requires mechanical addition of defining
> ONE_DIRECTION in each converter as part of the API contract with
> using the skeleton.c file. This is core cleanup work that will bear
> fruit!

It may be an indication the the "macro API" needs some more substantial
cleanup that makes it possible for the users of the API (definers of the
macros) to be less duplicative of each other.