Previously, "guix build starpu-fxt --with-input=openmpi=nmad" would lack
the '--enable-nmad' configure flag, because 'starpu-fxt' was calling:

  (package-arguments starpu)

and 'starpu' itself would still depend on 'openmpi', not 'nmad'.

* inria/storm.scm (starpu-configure-flags): New procedure.
(starpu-1.3)[arguments]: Use it.
(starpu+fxt)[arguments]: Use it.