This recipe shows how the superbuild pattern can be harnessed to muster the dependencies of your project. Let us take another look at the layout of the project:

.
├── CMakeLists.txt
├── external
│   └── upstream
│       ├── boost
│       │   └── CMakeLists.txt
│       └── CMakeLists.txt
└── src
    ├── CMakeLists.txt
    └── path-info.cpp

We have introduced four CMakeLists.txt files in the project source tree:

1. The root CMakeLists.txt will coordinate the superbuild.
2. The file in external/upstream will lead us to the boost leaf directory.
3. external/upstream/boost/CMakeLists.txt will take care of the Boost dependency.
4. Finally, the CMakeLists.txt under src will build our example code, which depends on Boost.

Let us start start the discussion with the external/upstream/boost/CMakeLists.txt file. Boost uses its own build system and hence we need to be slightly more verbose in our ExternalProject_Add to get everything set up correctly:

1. We keep the default values for the Directory options.
2. The Download step will download an archive of the desired version of Boost from their online server. We therefore set up the URL and the URL_HASH. The latter is needed to check the integrity of the downloaded archive. Since we do not wish to see a progress report of the download, we also set the DOWNLOAD_NO_PROGRESS option to true.
3. The Update step is left blank. If anything needs rebuilding, we do not want to download Boost once again.
4. The Configure step will use the native configuration tool provided by Boost in CONFIGURE_COMMAND. Since we want the superbuild to be cross-platform, we use the <SOURCE_DIR> variable to refer to the location for the unpacked sources:

CONFIGURE_COMMAND
  <SOURCE_DIR>/bootstrap.sh
  --with-toolset=${_toolset}
  --prefix=${STAGED_INSTALL_PREFIX}/boost
  ${_bootstrap_select_libraries}

5. The Build options declare an in-source build, by setting the BUILD_IN_SOURCE option to true. BUILD_COMMAND uses the Boost native build tool, b2. Since we will be doing an in-source build, we use again the <SOURCE_DIR> variable to refer to the location of the unpacked sources.

6. We then move on to the Install options. Boost manages installation using the same native build tool. In fact, the build and installation commands could easily be collapsed into one.
7. The Output logging options LOG_BUILD and LOG_INSTALL direct ExternalProject_Add to write log files for the build and installation operations, instead of outputting to screen.
8. Finally, the BUILD_BYPRODUCTS option lets ExternalProject_Add keep track of the freshly built Boost libraries in subsequent builds, even though their modification times might not get updated.

Once Boost has been built, the ${STAGED_INSTALL_PREFIX}/boost folder in the build directory will contain our desired libraries. We need to pass this information down to our project, whose build system is generated in src/CMakeLists.txt. To achieve this goal, we pass two additional CMAKE_CACHE_ARGS in the ExternalProject_Add in the root CMakeLists.txt:

1. CMAKE_INCLUDE_PATH: The path where CMake will look for C/C++ headers
2. CMAKE_LIBRARY_PATH: The path where CMake will look for libraries

By setting these variables to our freshly built installation of Boost, we ensure that the dependency will be properly picked up.