Use $(MAKE), not make

For the sake of systems where we want gmake.
diff --git a/tests/Makefile b/tests/Makefile
index 731dcd9..dee7fdb 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -92,7 +92,7 @@
 all: $(APPS)
 
 $(DEP):
-	make -C ../library
+	$(MAKE) -C ../library
 
 test_suite_aes.ecb.c : suites/test_suite_aes.function suites/test_suite_aes.ecb.data scripts/generate_code.pl suites/helpers.function suites/main_test.function
 	echo   "  Gen   $@"