Makefile: add -Wundef to the default CFLAGS

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Makefile-ccan

@@ -2,8 +2,8 @@
 # For simple projects you could just do:
 #	SRCFILES += $(wildcard ccan/*/*.c)
 
-#CCAN_CFLAGS=-g -O3 -Wall -Wstrict-prototypes -Wold-style-definition -Wmissing-prototypes -Wmissing-declarations
-CCAN_CFLAGS=-g3 -ggdb -Wall -Wstrict-prototypes -Wold-style-definition -Wmissing-prototypes -Wmissing-declarations -Wpointer-arith -Wwrite-strings -DCCAN_STR_DEBUG=1
+#CCAN_CFLAGS=-g -O3 -Wall -Wstrict-prototypes -Wold-style-definition -Wmissing-prototypes -Wmissing-declarations -Wpointer-arith -Wwrite-strings -Wundef -DCCAN_STR_DEBUG=1
+CCAN_CFLAGS=-g3 -ggdb -Wall -Wstrict-prototypes -Wold-style-definition -Wmissing-prototypes -Wmissing-declarations -Wpointer-arith -Wwrite-strings -Wundef -DCCAN_STR_DEBUG=1
 CFLAGS = $(CCAN_CFLAGS) -I. $(DEPGEN)