Revert "Only use gl_IGNORE_UNUSED_LIBRARIES with recent GSL"
This reverts commit 5bac91a5eee6eab297e1cb8e8309654c8a22a530. It's not the GSL version that causes this.
This reverts commit 5bac91a5eee6eab297e1cb8e8309654c8a22a530. It's not the GSL version that causes this.