Turn on gcc optimization (-O2)

This is required by distros that use Fortify.

Makefile

 CC = gcc
 AM_CFLAGS = -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2
-CFLAGS = -g -O0
+CFLAGS = -g -O2
 objects = mmc.o mmc_cmds.o
 
 CHECKFLAGS= -Wall -Werror -Wuninitialized -Wundef