Provide missing description for our FLEXIBLE_ARRAY_MEMBER defines

Provide missing description for our FLEXIBLE_ARRAY_MEMBER defines

Apparently autoheader throws a tantrum if you say 'AC_DEFINE([a],
[b])'.  Instead you must say 'AC_DEFINE([a], [b], [description of
a])'.  We were running into this in our replacement definitions for
FLEXIBLE_ARRAY_MEMBER, which were only getting built on autoconf
versions before 2.61 -- and this made us stop working with those
autoconf versinos.

Fixes bug 2430; bugfix on 0.2.3.1-alpha.