How to build and deploy an external library with CMake

I would like to use and deploy an external library in my program.

My project's structure is as follow (simplified):

ProjectDir
   |_ _build
   |_ CMakeLists.txt
   |_ Src
   |   |_ CMakeLists.txt
   |   |_ .h and .cpp files
   |_ ThirdParty
       |_ CMakeLists.txt
       |_ ExternalLib
          |_ includes
          |   |_ .h files
          |_ lib
              |_ Windows
              |    |_ .dll files
              |_ Linux
                   |_ .so files

For this library, I've only the include files and the binaries for Windows and Linux.

I'm able to find the includes files by using target_include_directories() in the CMakeLists.txt of the ThirdParty directory but I cannot build my program even by declaring the libraries with :

target_link_libraries(${CMAKE_PROJECT_NAME}
${CMAKE_CURRENT_SOURCE_DIR}/ThirdParty/ExternalLib/lib/Windows/extLib)

Note: this command is also in the CMakeLists.txt of ThirdParty directory.

I get the following error:

'../../../ThirdParty/ExternalLib/lib/Windows/extLib', needed by 'program' missing and no known rule to make it

I develop on a Windows platform with VS2022 and my program will be build and installed on a Linux server.

On Windows I've the latest version of CMake but on Linux the version is 3.16.

Can somebody help me in fixing this error?

You should import library as a target of you project. For instance something like

add_library(your_lib SHARED IMPORTED)
set_property(TARGET your_lib PROPERTY
             IMPORTED_LOCATION
             "${CMAKE_SOURCE_DIR}/ThirdParty/ExternalLib/lib/Linux/extLib.so") # to be adapted for windows
target_include_directories(your_lib
                       PUBLIC
                       ${CMAKE_SOURCE_DIR}/ThirdParty/ExternalLib/include)

then you should be able to use it as any other library in cmake:

target_link_libraries(yourExecutable PRIVATE your_lib)

see doc in details here: https://cmake.org/cmake/help/latest/guide/importing-exporting/index.html