Makefile, /Makefile: corrects make COVERAGE=...

Completes c661b652.
See #2630.

Makefile

 
-COVERAGE=
 VIDJIL_ALGO_SRC = algo/
 VIDJIL_BROWSER_SRC = browser/
 VIDJIL_SERVER_SRC = server/
 
-ifeq (${COVERAGE},1)
-	COVERAGE_OPTION=--coverage
-else
-	COVERAGE_OPTION=
-endif
-
 TEE = python tools/tee.py -v
 
 test_browser: unit_browser functional_browser

algo/Makefile

 CXX?=g++
-OPTIM=-O2 -DNDEBUG  $(COVERAGE) $(DEBUG)
+
+ifeq (${COVERAGE},1)
+	COVERAGE_OPTION=--coverage
+else
+	COVERAGE_OPTION=
+endif
+
+
+OPTIM=-O2 -DNDEBUG  $(COVERAGE_OPTION) $(DEBUG)
 override CXXFLAGS += -W -Wall -std=c++11 $(OPTIM)
 
 ## inspired from http://stackoverflow.com/questions/399850/best-compiler-warning-level-for-c-c-compilers/401276