Cloning gnudatalanguage gdl

Do not use the button of the GitHub repo, your cloning will be incomplete.

Since #1484 gdl uses git submodules.

To clone gdl, type :

git clone --recursive https://github.com/gnudatalanguage/gdl.git 

It will created a directory gdl.

To update an already existing (cloned) gdl repo, do:

• cd gdl (or the path where gdl was cloned)

• git submodule update --init

• or git pull --recurse-submodules

Then to compile gdl, see now https://github.com/gnudatalanguage/gdl/wiki/GDL-on-Linux#Compilation.