-GTEST_DIR = /usr/src/gtest
+GTEST_DIR ?= /usr/src/gtest
EIGEN_CXXFLAGS := $(shell pkg-config --cflags eigen3)
ifeq ($(EIGEN_CXXFLAGS),)
CC=gcc
CXX=g++
CXXFLAGS=-Wall -g -I$(GTEST_DIR)/include $(EIGEN_CXXFLAGS) $(GLEW_CXXFLAGS)
-LDFLAGS=-lSDL -lSDL_image -lGL -lrt -lpthread $(GLEW_LIBS)
+LDFLAGS=-lSDL -lSDL_image -lGL -lrt -lpthread -lpng $(GLEW_LIBS)
RANLIB=ranlib
ifeq ($(COVERAGE),1)