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.