openCV Troubleshooting (1) : undefined reference to `cv::xxxx`

I am compiling openCV project with cmake and I am getting errors like:

test.cpp:(.text+0xc0): undefined reference to `cv::imread(std::string const&, int)'
test.cpp:(.text+0x11f): undefined reference to `cv::_OutputArray::_OutputArray(cv::Mat&)'
test.cpp:(.text+0x138): undefined reference to `cv::_InputArray::_InputArray(cv::Mat const&)'
test.cpp:(.text+0x158): undefined reference to `cv::cvtColor(cv::_InputArray const&, cv::_OutputArray const&, int, int)'
test.cpp:(.text+0x180): undefined reference to `cv::_InputArray::_InputArray(cv::Mat const&)'

The following compiling script won't cause the issue.

g++ -o test_1 test_1.cpp `pkg-config opencv --cflags --libs`

But to fix cmake compile, we need to add the following red line to CMakeList.txt.

cmake_minimum_required(VERSION 2.8)
project( DisplayImage )
find_package( OpenCV REQUIRED )
add_executable( DisplayImage DisplayImage.cpp )
target_link_libraries( DisplayImage ${OpenCV_LIBS} )

ref:

https://stackoverflow.com/questions/24337932/cannot-get-opencv-to-compile-because-of-undefined-references

https://stackoverflow.com/questions/16782432/linker-can-not-find-opencv-libraries-when-using-cmake
https://docs.opencv.org/2.4/doc/tutorials/introduction/linux_gcc_cmake/linux_gcc_cmake.html