Don't hardcode optimization flags.

Index: Makefile
--- Makefile.orig
+++ Makefile
@@ -2,7 +2,7 @@ CC             ?= clang
 TTF_FONT_PATH  ?= res/Anonymous Pro.ttf
 CFLAGS         += -DTTF_FONT_PATH="\"$(TTF_FONT_PATH)\""
 CFLAGS         += -Wno-visibility -Wno-incompatible-pointer-types -Wall -Wextra
-CFLAGS         += -DINVERT_COLORS -DVT100 -O2
+CFLAGS         += -DINVERT_COLORS -DVT100
 LFLAGS         +=
 
 PROGRAM := 2048
