--- makefile.orig	2016-12-06 17:20:09 UTC
+++ makefile
@@ -1,4 +1,5 @@
-CXX = g++
+CXX ?= g++
+CXXFLAGS ?= -O2
 SRCS = library/preprocessor.cpp
 OUT = bin/preprocessor
 
@@ -15,6 +16,6 @@ clean:
 	rm -f $(OUT)
 
 $(OUT): $(SRCS)
-	$(CXX) -o $(OUT) -O2 -s $(SRCS)
+	$(CXX) -o $(OUT) $(CXXFLAGS) -s $(SRCS)
 
 .PHONY: all help clean