Fix up after 493d4d70d9968dc828bcbf29a2a8c9f042634827

As a result of 493d4d70d9968dc828bcbf29a2a8c9f042634827, `autoreconf` fails to run. Arguments that are supposed to be passed to `AC_ARG_ENABLE` were erroneously passed to `AS_HELP_STRING` instead.