The code for this recipe is available at https://github.com/dev-cafe/cmake-cookbook/tree/v1.0/chapter-08/recipe-03 and has a C example. The recipe is valid with CMake version 3.5 (and higher) and has been tested on GNU/Linux, macOS, and Windows.
The superbuild pattern can be used to manage fairly complex dependencies, for projects in all the languages supported by CMake. As demonstrated in the previous recipe, it is not necessary for the various subprojects to managed by CMake as well. In contrast to the previous recipe, the external subproject in this recipe will be a CMake project and will show how to download, build, and install the FFTW library using a superbuild. FFTW is a Fast Fourier Transform library, freely available at http://www.fftw.org.
