set PKG_CONFIG_PATH in cmake

You could set PKG_CONFIG_PATH with the CMake line:

set(ENV{PKG_CONFIG_PATH} "$ENV{PKG_CONFIG_PATH}:/my/path/to/pkgconfig")

I did this workaround in this file

Interestingly, it seems CMake 3.1 extends PKG_CONFIG_PATH with some CMake variable see: https://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=3df51470

After doing some research and a hint from the @OlivierM, I found the answer.

Here are the steps:

Method I :

CMAKE_PREFIX_PATH can be set to find the .pc files

set(CMAKE_PREFIX_PATH "${CMAKE_SOURCE_DIR}/libs/opencv-install")

Method II A second method is to use the PKG_CONFIG_PATH, which is a system environment variable to look for .pc files.

set(ENV{PKG_CONFIG_PATH} "${CMAKE_SOURCE_DIR}/libs/opencv-install/lib/pkgconfig")

Irrespective of which method you use,

For old (traditional) CMake:

find_package(PkgConfig REQUIRED)
pkg_search_module(PKG_OPENCV REQUIRED opencv)  # this looks for opencv.pc file

Please note that the PKG_OPENCV variable can be named anything. Whatever it is is named, its used as a prefix. For example if you name ABCD, then include directories will be ABCD_INCLUDE_DIRS

The variable PKG_OPENCV_INCLUDE_DIRS and PKG_OPENCV_LIBRARIES contains the header files (compile stage) and libraries (link stage) respectively.

One very important thing I noticed was that the variable PKG_OPENCV_LIBRARIES just provides the libraries and not the library path during the link stage. In order to use the library path as well in one command, one has to use

PKG_OPENCV_LDFLAGS 

This variable contains the library path as well as all the libraries listed in the package config file.

for examaple:

include_directories(${PKG_OPENCV_INCLUDE_DIRS})
target_link_libraries (FINAL_BINARY ${PKG_OPENCV_LDFLAGS})

For modern CMake:

In modern CMake we don't want variables, we want targets.

find_package(PkgConfig REQUIRED)
# this looks for opencv.pc file and creates a new target
# IMPORTED_TARGET requires CMake >= 3.6.3
pkg_search_module(PKG_OPENCV REQUIRED IMPORTED_TARGET opencv)

All variables will still be created for backwards compatibility, but IMPORTED_TARGET will create a target you can use in your project which will automatically propagate all of its build and usage requirements:

target_link_libraries(my_proj PRIVATE PkgConfig::PKG_OPENCV)