Package opencv was not found in the pkg-config search path.

安装好后opencv后执行下面这条语句的时候出错：

pkg-config --cflags opencv

Package opencv was not found in the pkg-config search path.
Perhaps you should add the directory containing `opencv.pc'
to the PKG_CONFIG_PATH environment variable
No package 'opencv' found

经过网上详细查询资料后，是缺失了opencv.pc这个配置信息文件，故解决方法就是添加这个文件然后将其导入到环境变量中，具体操作如下：

首先创建opencv.pc文件，这里要注意它的路径信息：

cd /usr/local/lib
sudo mkdir pkgconfig
cd pkgconfig
sudo touch opencv.pc

然后在opencv.pc中添加以下信息，注意这些信息需要与自己安装opencv时的库路径对应：

prefix=/usr/local
exec_prefix=${prefix}
includedir=${prefix}/include
libdir=${exec_prefix}/lib

Name: opencv
Description: The opencv library
Version:4.0.1
Cflags: -I${includedir}/opencv4
Libs: -L${libdir} -lopencv_shape -lopencv_stitching -lopencv_objdetect -lopencv_superres -lopencv_videostab -lopencv_calib3d -lopencv_features2d -lopencv_highgui -lopencv_videoio -lopencv_imgcodecs -lopencv_video -lopencv_photo -lopencv_ml -lopencv_imgproc -lopencv_flann  -lopencv_core
~                                               

保存退出，然后将文件导入到环境变量：

export  PKG_CONFIG_PATH=/usr/local/lib/pkgconfig

至此就配置好opencv.pc啦～
再执行 pkg-config --cflags --libs opencv时输出结果如下：

-I/usr/local/include/opencv4 -L/usr/local/lib \
-lopencv_shape -lopencv_stitching -lopencv_objdetect \
-lopencv_superres -lopencv_videostab -lopencv_calib3d \
 -lopencv_features2d -lopencv_highgui -lopencv_videoio \
 -lopencv_imgcodecs -lopencv_video -lopencv_photo -lopencv_ml \
 -lopencv_imgproc -lopencv_flann -lopencv_core

下面是opencv官方例程(opencv-4.0.1/samples/cpp/example_cmake)里的代码文件和Makefile文件：
example.cpp:

#include "opencv2/core.hpp"
#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"
#include "opencv2/videoio.hpp"
#include <iostream>

using namespace cv;
using namespace std;

void drawText(Mat & image);

int main()
{
    cout << "Built with OpenCV " << CV_VERSION << endl;
    Mat image;
    VideoCapture capture;
    capture.open(0);
    if(capture.isOpened())
    {
        cout << "Capture is opened" << endl;
        for(;;)
        {
            capture >> image;
            if(image.empty())
                break;
            drawText(image);
            imshow("Sample", image);
            if(waitKey(10) >= 0)
                break;
        }
    }
    else
    {
        cout << "No capture" << endl;
        image = Mat::zeros(480, 640, CV_8UC1);
        drawText(image);
        imshow("Sample", image);
        waitKey(0);
    }
    return 0;
}

void drawText(Mat & image)
{
    putText(image, "Hello OpenCV",
            Point(20, 50),
            FONT_HERSHEY_COMPLEX, 1, // font face and scale
            Scalar(255, 255, 255), // white
            1, LINE_AA); // line thickness and type
}

Makefile:

CXX ?= g++

CXXFLAGS += -c -Wall $(shell pkg-config --cflags opencv)
LDFLAGS += $(shell pkg-config --libs --static opencv)

all: opencv_example

opencv_example: example.o; $(CXX) $< -o $@ $(LDFLAGS)

%.o: %.cpp; $(CXX) $< -o $@ $(CXXFLAGS)

clean: ; rm -f example.o opencv_example

我们在该例程下执行make来测试，编译过程如下：

g++ example.cpp -o example.o -c -Wall -I/usr/local/include/opencv4
g++ example.o -o opencv_example -L/usr/local/lib -lopencv_shape -lopencv_stitching -lopencv_objdetect -lopencv_superres -lopencv_videostab -lopencv_calib3d -lopencv_features2d -lopencv_highgui -lopencv_videoio -lopencv_imgcodecs -lopencv_video -lopencv_photo -lopencv_ml -lopencv_imgproc -lopencv_flann -lopencv_core

编译过程没有问题且生成了可执行文件opencv_example，执行起来没有问题，但是报了一点点小错误：

Gtk-Message: 06:57:54.502: Failed to load module "canberra-gtk-module"

这不是程序代码有问题，也不是编译问题，只需要安装一下 libcanberra-gtk-module即可：

sudo apt-get install libcanberra-gtk-module

最终运行结果如下：