BLD: change gsl to build-only dependency

We do static link with `gsl` so it is not needed at runtime.

Author: pavoljuhas

conda-recipe/meta.yaml

@@ -18,11 +18,11 @@ build:
 requirements:
   build:
     - {{ compiler('cxx') }}     # [py3k or not win]
+    - gsl {{ gsl }}
   host:
     - python {{ python }}
     - pip
     - setuptools
-    - gsl {{ gsl }}
 
   run:
     - python