gsl: New version 1.15
gsl: New version 1.15

Import GSL formula at version 1.15 from upstream linux/homebrew.
This matches current version in Cadfael 1.1.1

SuperNEMO-DBD/homebrew-cadfael,SuperNEMO-DBD/homebrew-cadfael,xgarrido/homebrew-cadfael,xgarrido/homebrew-cadfael