fix format warnings
Enable format security, and fix the warning caused by printing
with string for format.
diff --git a/Makefile b/Makefile
index 543d9e8..9dbb29f 100644
--- a/Makefile
+++ b/Makefile
@@ -31,7 +31,7 @@
DEFINES += -D_GNU_SOURCE
CCOPTS = -O2
WFLAGS := -Wall -Wstrict-prototypes -Wmissing-prototypes
-WFLAGS += -Wmissing-declarations -Wold-style-definition
+WFLAGS += -Wmissing-declarations -Wold-style-definition -Wformat=2
CFLAGS = $(WFLAGS) $(CCOPTS) -I../include $(DEFINES)
YACCFLAGS = -d -t -v