For the official build info look at the combined refu language repo.
Alternatively if you are feeling adventurous and want to clone the latest versions of each separate repository yourself do the following, to use the build system repo to pull the other repos.
mkdir refu
cd refu
git clone git@github.com:LefterisJP/refubuildsystem.git build_system
ln -s build_system/SConstruct SConstruct
scons update_all
After that you can build the different repos and their tests with scons commands like
scons clib
scons clib_tests
scons core
scons lang_tests