After a sucessful git clone :

git submodule init
git submodule update
git submodule foreach git checkout master

Now you have a local copy of submodule repositories.
If you want to update all submodule repositories to lastest version, run this command:

git submodule foreach git pull

To build the project :

mkdir build
cd build
cmake ..
make
sudo make install

The main CMakeLists.txt use submodules CMakeLists.txt in order to compile all submodule project.