licenses: Add CC-BY-SA and CC-BY.

* guix/licenses.scm (cc-by-sa4.0, cc-by3.0): New variables.

guix/licenses.scm

@@ -30,6 +30,7 @@
             non-copyleft
             bsd-style                             ;deprecated!
             cc0
+            cc-by-sa4.0 cc-by3.0
             cddl1.0
             cecill-c
             artistic2.0 clarified-artistic
@@ -137,6 +138,16 @@ at URI, which may be a file:// URI pointing the package's tree."
            "http://directory.fsf.org/wiki/License:CC0"
            "http://www.gnu.org/licenses/license-list.html#CC0"))
 
+(define cc-by-sa4.0
+  (license "CC-BY-SA 4.0"
+           "http://creativecommons.org/licenses/by-sa/4.0/"
+           "Creative Commons Attribution-ShareAlike 4.0 International"))
+
+(define cc-by3.0
+  (license "CC-BY 3.0"
+           "http://creativecommons.org/licenses/by/3.0/"
+           "Creative Commons Attribution 3.0 Unported"))
+
 (define cddl1.0
   (license "CDDL 1.0"
            "http://directory.fsf.org/wiki/License:CDDLv1.0"