gsl: 1.16 -> 2.1

Release notes :
– gsl-2.0: https://savannah.gnu.org/forum/forum.php?forum_id=8392
— gsl-2.1: https://savannah.gnu.org/forum/forum.php?forum_id=8408

The bug-39055.patch is removed since it seems to be integrated in the
2.1 release.