Compile with -g -O2 by default

Change default CFLAGS (CFLAGS not overriden and no CFLAGS in
environment) to add debugging symbol and optimize the generated code.

Makefile

@@ -15,7 +15,7 @@
 #
 
 VERSION = $(shell awk 'END { print $$1 }' VERSION)
-CFLAGS ?= -Wall -Wextra
+CFLAGS ?= -Wall -Wextra -g -O2
 CFLAGS += -DVERSION=\"$(VERSION)\"
 
 # Note: assignments are ignored for variable overriden on command line VAR=value