Makefile: Add a commented out CFLAGS for developing

- Use -DDEBUG
- Use -O0 instead of -Os, since Os includes -O2, a thing that makes
  using a debugger harder due to a changed order of computation and
  an inability to inspect variables that got optimized.

Makefile

@@ -3,8 +3,10 @@
 # license that can be found in the LICENSE file.
 
 CC=gcc
-# Add -DDEBUG to CFLAGS when developing/testing
+
 CFLAGS=-g -Wall -Os
+# Comment out when developing/testing
+#CFLAGS=-DDEBUG -g -Wall -O0
 
 TARG=re
 OFILES=\