How to include third party tools with cmake?

I have been working on a CPP project but was using configuration on visual studio IDE. Now, I wanted to use a build system generator CMake. It is a little difficult to getting started with this.

I am trying to add cppunit third-party tool for my testing. For that, I have added include and lib file in a folder third_party. But not sure how to include it in the CMakeLists.txt.

Please find CMakeList.txt

# CMake version setting
cmake_minimum_required(VERSION 3.8)
if(${CMAKE_VERSION} VERSION_LESS 3.19)
    cmake_policy(VERSION ${CMAKE_MAJOR_VERSION}.${CMAKE_MINOR_VERSION})
else()
    cmake_policy(VERSION 3.19)
endif()

# Set project name and version
project(myproject
    VERSION 1.0
    DESCRIPTION "Setup cmake build system"
    LANGUAGES CXX
)

# Third party dependencies
set(THIRD_PARTY_DIR "${PROJECT_SOURCE_DIR}/third_party")

# CPP unit
set(CPP_UNIT_LIB_NAME "cppunit")
set(CPP_UNIT_VERSION "1.13.2")
set(CPP_UNIT_DIR "${THIRD_PARTY_DIR}/${CPP_UNIT_LIB_NAME}/${CPP_UNIT_VERSION}")

# NOT sure what to do here
# add_subdirectory(${CPP_UNIT_DIR})
# target_include_directories(${PROJECT_NAME} PRIVATE "${CPP_UNIT_DIR}/include/cppunit")
# target_link_libraries(${PROJECT_NAME}  ${CPP_UNIT_LIB_NAME} ${CPP_UNIT_LIBRARIES})
# target_compile_definitions(${PROJECT_NAME}  PRIVATE "CPP_UNIT_INCLUDE_NONE")

add_subdirectory(src)

Please find the snap-shot of the folder structure

add_subdirectory(${CPP_UNIT_DIR}) will look in the directory specificied for a CMakeLists.txt and since CppUnit has a CMakeLists.txt file (https://github.com/Ultimaker/CppUnit/blob/master/CMakeLists.txt) it will build the library specified add_library(cppunit STATIC ${Sources}) which is cppunit.

And then when you specify your target to build, you can link in cppunit using target_link_libraries(your_target cppunit). But you need to create your target such as by using add_executable(one two.cpp three.h) which creates the target one.

https://cliutils.gitlab.io/modern-cmake/ is a good introductory resource for CMake. And there are different ways to bring in external projects such as through a git submodule.

If you have a CMakeLists.txt file in the src subdirectory, where you create a target you can link in cppunit.

As for the comment about include_directories, it is generally considered good practice to use target_include_directories instead, see What is the difference between include_directories and target_include_directories in CMake? and the above linked resource for more.