Use DISTCHECK_CONFIGURE_FLAGS to make recursive distcheck work again.

Use DISTCHECK_CONFIGURE_FLAGS to make recursive distcheck work again.

DISTCHECK_CONFIGURE_FLAGS is a magic automake macro for the
distcheck target recursive tests between scripted runs.
we use it to perform the same duty between our nested scripts.

We used it earlier but lost it through a bit of forgetfulness. Documented
now to prevent that happening again.