Makefile: use -Os instead of -O2

-Os provides -O2 minus some optimizations that would
increase size, plus optimizations that would decreaze
size, plus -Os usually takes caching into account.

    -Os Optimize for size.  -Os enables all -O2 optimizations that do not
        typically increase code size.  It also performs further optimizations
        designed to reduce code size.

Makefile

@@ -1,6 +1,6 @@
 CC	?= gcc
 STRIP ?= strip
-CFLAGS ?= -O2
+CFLAGS ?= -Os
 LDFLAGS += -lxcb
 CFDEBUG = -g3 -pedantic -Wall -Wunused-parameter -Wlong-long\
 		  -Wsign-conversion -Wconversion -Wimplicit-function-declaration