Missing submodules

Unfortunately GitHub doesn't include the required git submodules available in the repository in their zip / tar.gz packages, so you need to download the related source manually separately. This leads to the same error below also if you haven't cloned the repository with the --recursive / recurse-submodules switch, resulting with the error:

CMake Error at lib/CMakeLists.txt:NNN (add_subdirectory):
  The source directory
  does not contain a CMakeLists.txt file.
-- Configuring incomplete, errors occurred!

run git submodule update --init --recursive from the project directory.