fix: apply build FLAGS correctly

A typo in the dependency variable program_obj_files caused make to fall back to implicit/default rules. FLAGS was ignored because the default behavior is to use CFLAGS.

makefile

@@ -29,7 +29,7 @@ program_obj_files = stack.o bintree.o numbers.o timer.o highscore.o
 doble : main.o $(program_obj_files)
 	$(CC) $(FLAGS) $^ -o doble
 
-$(program_obj_filesobj_files): %.o: %.c
+$(program_obj_files): %.o: %.c
 	$(CC) -c $(FLAGS) $^ -o $@
 
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