Enforce printf format literals checking

Change-Id: Ifcba2251a723770e8bce12e39beda8236c814c42
Reviewed-on: https://weave-review.googlesource.com/2320
Reviewed-by: Alex Vakulenko <avakulenko@google.com>
diff --git a/Makefile b/Makefile
index 512c2a6..4ea625f 100644
--- a/Makefile
+++ b/Makefile
@@ -26,9 +26,9 @@
 	-Wall \
 	-Werror \
 	-Wextra \
+	-Wformat=2 \
 	-Wl,--exclude-libs,ALL \
 	-Wno-char-subscripts \
-	-Wno-format-nonliteral \
 	-Wno-missing-field-initializers \
 	-Wno-unused-local-typedefs \
 	-Wno-unused-parameter \