Dev-C++ 5.11 with MinGW have linking problem

I did try to compile a OpenCV-Project with MinGW (under Windows 10). The OpenCV-library had compiled without problem and I try to do couple of test.

main3.cpp

#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgcodecs/imgcodecs.hpp"
#include <iostream>

using namespace cv;
using namespace std;

int main(int argc, char** argv)
{
  Mat im = imread(argc == 2 ? argv[1] : "lena.jpg", 1);
  if (im.empty())
  {
    cout << "Cannot open image!" << endl;
    return -1;
  }

  imshow("image", im);
  waitKey(0);

  return 0;
}

I have follows Makefile:

# Project: Proj400-v3
# Makefile created by Dev-C++ 5.11

CPP      = g++.exe
CC       = gcc.exe
WINDRES  = windres.exe
OBJ      = obj/main3.o
LINKOBJ  = obj/main3.o
LIBS     = -L"C:/RD/TMP/Dev-Cpp/MinGW64/lib32" -L"C:/RD/TMP/Work/entw/OCV-SVD/ocv4.0.0/lib" -lopencv_core400 -lopencv_imgcodecs400 -lopencv_highgui400 -m32
INCS     = -I"C:/RD/TMP/Dev-Cpp/MinGW64/include" -I"C:/RD/TMP/Dev-Cpp/MinGW64/x86_64-w64-mingw32/include" -I"C:/RD/TMP/Dev-Cpp/MinGW64/lib/gcc/x86_64-w64-mingw32/4.9.2/include" -I"C:/RD/TMP/Work/entw/OCV-SVD/ocv4.0.0/inc" -I"C:/RD/TMP/Work/entw/OCV-SVD/proj4.0.0.0"
CXXINCS  = -I"C:/RD/TMP/Work/entw/OCV-SVD/ocv4.0.0/inc" -I"C:/RD/TMP/Work/entw/OCV-SVD/proj4.0.0.0"
BIN      = obj/Proj400-v3.exe
CXXFLAGS = $(CXXINCS) -m32 -std=c++11 -lopencv_core400 -lopencv_imgcodecs400 -lopencv_highgui400
CFLAGS   = $(INCS) -m32
RM       = rm.exe -f

.PHONY: all all-before all-after clean clean-custom

all: all-before $(BIN) all-after

clean: clean-custom
    ${RM} $(OBJ) $(BIN)

$(BIN): $(OBJ)
    $(CPP) $(LINKOBJ) -o $(BIN) $(LIBS)

obj/main3.o: main3.cpp
    $(CPP) -c main3.cpp -o obj/main3.o $(CXXFLAGS)

All compiled and linked without mistakes nad run fine. I try to expand the test with new file 'second3.cpp'.

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/imgproc/imgproc_c.h>


extern "C" {
}

New the Makefile:

# Project: Proj400-v3
# Makefile created by Dev-C++ 5.11

CPP      = g++.exe
CC       = gcc.exe
WINDRES  = windres.exe
OBJ      = obj/main3.o obj/second3.o
LINKOBJ  = obj/main3.o obj/second3.o
LIBS     = -L"C:/RD/TMP/Dev-Cpp/MinGW64/lib32" -L"C:/RD/TMP/Work/entw/OCV-SVD/ocv4.0.0/lib" -lopencv_core400 -lopencv_imgcodecs400 -lopencv_highgui400 -m32
INCS     = -I"C:/RD/TMP/Dev-Cpp/MinGW64/include" -I"C:/RD/TMP/Dev-Cpp/MinGW64/x86_64-w64-mingw32/include" -I"C:/RD/TMP/Dev-Cpp/MinGW64/lib/gcc/x86_64-w64-mingw32/4.9.2/include" -I"C:/RD/TMP/Work/entw/OCV-SVD/ocv4.0.0/inc" -I"C:/RD/TMP/Work/entw/OCV-SVD/proj4.0.0.0"
CXXINCS  = -I"C:/RD/TMP/Work/entw/OCV-SVD/ocv4.0.0/inc" -I"C:/RD/TMP/Work/entw/OCV-SVD/proj4.0.0.0"
BIN      = obj/Proj400-v3.exe
CXXFLAGS = $(CXXINCS) -m32 -std=c++11 -lopencv_core400 -lopencv_imgcodecs400 -lopencv_highgui400
CFLAGS   = $(INCS) -m32
RM       = rm.exe -f

.PHONY: all all-before all-after clean clean-custom

all: all-before $(BIN) all-after

clean: clean-custom
    ${RM} $(OBJ) $(BIN)

$(BIN): $(OBJ)
    $(CPP) $(LINKOBJ) -o $(BIN) $(LIBS)

obj/main3.o: main3.cpp
    $(CPP) -c main3.cpp -o obj/main3.o $(CXXFLAGS)

obj/second3.o: second3.cpp
    $(CPP) -c second3.cpp -o obj/second3.o $(CXXFLAGS) 

After compiling I have follows list of mistakes:

- Command: mingw32-make.exe -f "C:\RD\TMP\Work\entw\OCV-SVD\proj4.0.0.0\Makefile.win" all

g++.exe obj/main3.o obj/second3.o -o obj/Proj400-v3.exe -L"C:/RD/TMP/Dev-Cpp/MinGW64/lib32" -L"C:/RD/TMP/Work/entw/OCV-SVD/ocv4.0.0/lib" -lopencv_core400 -lopencv_imgcodecs400 -lopencv_highgui400 -m32

obj/main3.o:main3.cpp:(.text+0xc3): undefined reference to `cv::imread(std::string const&, int)'
obj/main3.o:main3.cpp:(.text+0x181): undefined reference to `cv::imshow(std::string const&, cv::_InputArray const&)'
collect2.exe: error: ld returned 1 exit status

C:\RD\TMP\Work\entw\OCV-SVD\proj4.0.0.0\Makefile.win:25: recipe for target 'obj/Proj400-v3.exe' failed

mingw32-make.exe: *** [obj/Proj400-v3.exe] Error 1

Can some body explane me, what is wrong? Why with one source in the project it work fine und with many files in the project I have 'undefined reference'?

PS: If I change the project from two source-files to one-source, but do not remove the obj/second3.o, I have again problem with 'undefined reference'.