Make optimization flags configurable

Makefile

@@ -6,8 +6,10 @@ STRIP ?= strip
 PKG_CONFIG ?= pkg-config
 INSTALL ?= install
 
-CFLAGS ?= -O3
+CFLAGS_OPTIMIZATION ?= -O3
+
 CFLAGS += -Wall -Wextra -Wno-unused-parameter
+CFLAGS += $(CFLAGS_OPTIMIZATION)
 
 ifeq ($(shell $(PKG_CONFIG) ncursesw && echo 1),1)
 	CFLAGS += $(shell $(PKG_CONFIG) --cflags ncursesw)