Since building the constify_plugin now depends on CONFIG_PAX_CONSTIFY_PLUGIN (a change from 3.8.3 to 3.8.4), the plugin doesn't get built -but, strangely enough(?), still expected to be available- when CONFIG_GRKERNSEC is unset.
Can you try this on 3.8.7 which I just added to the tree?
3.8.7 builds fine (and so did 3.8.6 and 3.8.5). The bug only concerns/concerned 3.8.4...