--- vcflib/Makefile.orig	2018-03-23 18:36:07 UTC
+++ vcflib/Makefile
@@ -136,8 +136,8 @@ all: $(OBJECTS) $(BINS)
 
 GIT_VERSION := $(shell git describe --abbrev=4 --dirty --always)
 
-CXX = g++
-CXXFLAGS = -O3 -D_FILE_OFFSET_BITS=64 -std=c++0x 
+CXX ?= g++
+CXXFLAGS += -D_FILE_OFFSET_BITS=64 -std=c++0x 
 #CXXFLAGS = -O2
 #CXXFLAGS = -pedantic -Wall -Wshadow -Wpointer-arith -Wcast-qual