do not echo coverage-generating commands

Makefile

@@ -9,5 +9,5 @@ clean:
 test:
 	./waf build --targets tests
 	./build/tests
-	mkdir -p coverage
-	-(cd build; gcovr --object-directory=src/core --root=.. -o ../coverage/index.html --html --html-details --gcov-filter='.*src#core.*')
+	@mkdir -p coverage
+	-@(cd build; gcovr --object-directory=src/core --root=.. -o ../coverage/index.html --html --html-details --gcov-filter='.*src#core.*')