Use the correct case for -Ddebug_<object>

src/Makefile

 #
 COMPILE_c	= $(CC) $(CFLAGS) $(CFLAGS_c) $(OBJ_CFLAGS)
 RULE_c		= $(COMPILE_c) -c $< -o $@
-RULE_c_to_dbg.o	= $(COMPILE_c) -DDEBUG_$(OBJECT) -c $< -o $@
+RULE_c_to_dbg.o	= $(COMPILE_c) -Ddebug_$(OBJECT) -c $< -o $@
 RULE_c_to_c	= $(COMPILE_c) -E -c $< > $@
 RULE_c_to_s	= $(COMPILE_c) -S -c $< -o $@