fix Makefile

4 jobs for master in 7 minutes and 13 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed stdpp-coq8.6 #3461
fp-timing

00:01:49

passed stdpp-coq8.6.1 #3460
fp-timing

00:02:11

passed stdpp-coq8.7 #3459
fp-timing

00:05:08

 
  Opam
passed opam #3462

00:00:13