Undefined references for libusb library

Question

I am trying to use GoogleTest to test a simple function, but as I run make in my build folder, the compiler throws Undefined Reference error messages at me for libusb library. I need to add libusb library in GTEST_SRCS_ part of the makefile but I get same error. I am new to unit testing so, I do not know what should I do? How to build gtest with libusb library?

Makefile

GTEST_DIR= /home/sahin/googletest/googletest
LIBUSB_DIR= /usr/include/libusb-1.0

USER_DIR=.
LIBS += -L/usr/lib -lusb-1.0 -L/usr/lib/x86_64-linux-gnu/libusb-1.0.so -L./ 

CPPFLAGS += -I$(GTEST_DIR)/include -I$(LIBUSB_DIR)

CXXFLAGS += -g -Wall -Wextra -pthread

OBJECTS = main.o test.o
TESTS = test

GTEST_HEADERS = $(GTEST_DIR)/include/gtest/*.h \
                $(GTEST_DIR)/include/gtest/internal/*.h
LIBUSB_HEADERS = $(LIBUSB_DIR)/*.h

all : $(TESTS)

clean :
    rm -f $(TESTS) gtest.a gtest_main.a *.o

GTEST_SRCS_ = $(GTEST_DIR)/src/*.cc $(GTEST_DIR)/src/*.h $(GTEST_HEADERS)

gtest-all.o : $(GTEST_SRCS_)
    $(CXX) $(CPPFLAGS) -I$(GTEST_DIR) $(CXXFLAGS) -c \
            $(GTEST_DIR)/src/gtest-all.cc

gtest_main.o : $(GTEST_SRCS_)
    $(CXX) $(CPPFLAGS) -I$(GTEST_DIR) $(CXXFLAGS) -c \
            $(GTEST_DIR)/src/gtest_main.cc

gtest.a : gtest-all.o
    $(AR) $(ARFLAGS) $@ $^

gtest_main.a : gtest-all.o gtest_main.o
    $(AR) $(ARFLAGS) $@ $^

test.o : $(USER_DIR)/test.cpp $(GTEST_HEADERS)
    $(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $(USER_DIR)/test.cpp

test : test.o gtest_main.a
    $(CXX) $(CPPFLAGS) $(CXXFLAGS) -lpthread $^ -o $@