Add dependency for cmaybe.h

Makefile

@@ -1,5 +1,8 @@
 all: average
 
+average: average.c cmaybe.h
+	$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) -o $@ $<
+
 clean:
 	rm -f average
 

cmaybe.h

@@ -1,3 +1,6 @@
+#ifndef __CMAYBE_H__
+#define __CMAYBE_H__
+
 #define MAYBE_TYPE(name, type) struct maybe_##name {type value; char is_value;}
 #define MAYBE(name) struct maybe_##name
 
@@ -15,3 +18,5 @@
 #define IS_VALUE(x) if((x).is_value)
 #define IS_NOT_VALUE(x) if(!(x).is_value)
 #define VALUE(x) (x).value
+
+#endif