Not able to include a library in makefile - makefile

Question

I am kind of new in the use of make.
I have a library written in C which is in the folder folder1
Where these files are there is also a makefile which is the following:
PREFIX ?= /usr/local
CC = gcc
AR = ar
CFLAGS = -std=gnu99 -fPIC -Wall -Wno-unused-parameter -Wno-unused-function -I. -O4
APRILTAG_SRCS := $(shell ls *.c common/*.c)
APRILTAG_HEADERS := $(shell ls *.h common/*.h /usr/include/flycapture/*.h)
APRILTAG_OBJS := $(APRILTAG_SRCS:%.c=%.o)
TARGETS := libapriltag.a libapriltag.so
.PHONY: all
all: $(TARGETS)
#$(MAKE) -C example all
.PHONY: install
install: libapriltag.so
#chmod +x install.sh
#./install.sh $(PREFIX)/lib libapriltag.so
#./install.sh $(PREFIX)/include/apriltag $(APRILTAG_HEADERS)
#sed 's:^prefix=$$:prefix=$(PREFIX):' < apriltag.pc.in > apriltag.pc
#./install.sh $(PREFIX)/lib/pkgconfig apriltag.pc
#rm apriltag.pc
#ldconfig
libapriltag.a: $(APRILTAG_OBJS)
#echo " [$#]"
#$(AR) -cq $# $(APRILTAG_OBJS)
libapriltag.so: $(APRILTAG_OBJS)
#echo " [$#]"
#$(CC) -fPIC -shared -o $# $^
%.o: %.c
#echo " $#"
#$(CC) -o $# -c $< $(CFLAGS)
.PHONY: clean
clean:
#rm -rf *.o common/*.o $(TARGETS)
#$(MAKE) -C example clean
Then, in the folder example I have some example which I can just run by giving the following command in the terminal:
./opencv_demo
or
./apriltag_demo
In this folder example I have another makefile which is the following:
CC = gcc
CXX = g++
CPPFLAGS = -I..
CFLAGS = -std=gnu99 -Wall -Wno-unused-parameter -Wno-unused-function -O4
CXXFLAGS = -Wall -O4
LDFLAGS = -Lusr/include/flycapture -lpthread -lm
INC_DIR = /usr/include/flycapture
TARGETS := apriltag_demo opencv_demo
.PHONY: all
all: apriltag_demo opencv_demo
apriltag_demo: apriltag_demo.o ../libapriltag.a
#echo " [$#]"
#$(CC) -o $# $^ $(LDFLAGS)
opencv_demo: opencv_demo.o ../libapriltag.a /usr/include/flycapture/Error.h
#echo " [$#]"
#$(CXX) -o $# $^ $(LDFLAGS) `pkg-config --libs opencv`
%.o: %.c
#echo " $#"
#$(CC) -o $# -c $< $(CFLAGS) $(CPPFLAGS)
%.o: %.cc
#echo " $#"
#$(CXX) -o $# -c $< $(CXXFLAGS) $(CPPFLAGS)
.PHONY: clean
clean:
#rm -rf *.o $(TARGETS)
If I do make in the folder1 I get the following error:
opencv_demo.o: In function `main':
opencv_demo.cc:(.text.startup+0x24e): undefined reference to `FlyCapture2::Error::Error()'
opencv_demo.cc:(.text.startup+0x2a7): undefined reference to `FlyCapture2::Error::~Error()'
opencv_demo.cc:(.text.startup+0x11a0): undefined reference to `FlyCapture2::Error::~Error()'
collect2: error: ld returned 1 exit status
make[1]: *** [opencv_demo] Error 1
make[1]: Leaving directory `/home/fschiano/Repositories/apriltag2/example'
make: *** [all] Error 2
If I am not wrong this error tells me that I did not include a library.
In the file opencv_demo.cc I did:
#include "flycapture/FlyCapture2.h"
#include "flycapture/Error.h"
using namespace FlyCapture2;
Headers which are in /usr/include/flycapture a folder which contains all the headers needed for what I need in the file opencv_demo.cc.
I don't know why it is not working, someone is able to help me?
I think I should write in the makefile where to search for the libraries but I don't know how.
I tried to write the following things in the makefile separately but they are not working :
LDFLAGS = -Lusr/include/flycapture -lpthread -lm
INC_DIR = /usr/include/flycapture
DEPS = $(INC_DIR)/Error.h