Fix statically allocated struct with FLEXIBLE_ARRAY_MEMBER member.

Fix statically allocated struct with FLEXIBLE_ARRAY_MEMBER member.

clang complains about this, not unreasonably, so define another struct
that's explicitly for a WordEntryPos with exactly one element.

While at it, get rid of pretty dubious use of a static variable for
more than one purpose --- if it were being treated as const maybe
I'd be okay with this, but it isn't.