[Makefile] fix make -j (dimacs.mll)
[Makefile] fix make -j (dimacs.mll)

jpdeplaix/why3,ssaavedra/why3,jpdeplaix/why3,ssaavedra/why3,jpdeplaix/why3,douglas-larocca/why3,douglas-larocca/why3,douglas-larocca/why3,ssaavedra/why3