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_stitching -lopencv_objdetect 
-lopencv_calib3d 
 -lopencv_features2d -lopencv_highgui -lopencv_videoio 
 -lopencv_imgcodecs -lopencv_video -lopencv_photo -lopencv_ml 
 -lopencv_imgproc -lopencv_flann -lopencv_core