Allow to set CFLAGS/LDFLAGS to point to MC version of simgrid

tests/model-checking/Makefile

@@ -1,6 +1,6 @@
 # StarPU --- Runtime system for heterogeneous multicore architectures.
 #
-# Copyright (C) 2017  Université de Bordeaux
+# Copyright (C) 2017, 2019  Université de Bordeaux
 #
 # StarPU is free software; you can redistribute it and/or modify
 # it under the terms of the GNU Lesser General Public License as published by
@@ -15,8 +15,8 @@
 
 STARPU=../../
 CPPFLAGS=-I$(STARPU)/src -I$(STARPU)/include -I.
-CFLAGS=-Wall -Wextra -g -DNOCONFIG
-LDFLAGS=-lsimgrid
+CFLAGS+=-Wall -Wextra -g -DNOCONFIG
+LDFLAGS+=-lsimgrid
 
 MC_FLAGS=--cfg=model-check/reduction:none