We've now verified that it's reliable for the user to override CFLAGS
on the command line, so just make the user do to get a debug build.
# Default FLAGS, (can be overriden by user such as "make CFLAGS=-O2")
WARN_FLAGS=-Wall -Wextra -Wmissing-declarations -Wwrite-strings -Wswitch-enum
-CFLAGS=-g -O0
+CFLAGS=-O2
# Additional flags that we will append to whatever the user set.
# These aren't intended for the user to manipulate.