[Patch] Fix ONE_DIRECTION undef warnings.

[Roland McGrath]

> As Steve suggested in another email it might make sense to have a single
> header define "macro API" defaults, that make sense for 99% of the
> configurations. It would make it easy to adjust them en-masse instead of
> rafactoring the changes across the 54 source files that use it.

The obvious way to do this makes all those source files places where typos
could go unnoticed.  We should think hard about ways to reduce that danger.
IMHO this is a stronger motivator than avoiding duplicate changes that can
be easily mechanized.  The former leads to silent bugs gone unnoticed,
which is a risk of real user harm that might not be noticed until much
later.  The latter leads to build errors/warnings that alert one early to
the need to do the annoying mechanically-duplicative change, which is a
certainty of immediate slight maintainer annoyance that happens immediately
to the maintainer who instigated the change.