Turn more licenses into lib.licenses style

Should eval cleanly, as far as -A tarball tells me.

Relevant: issue #2999, issue #739

pkgs/development/libraries/slib/default.nix

@@ -52,7 +52,7 @@ stdenv.mkDerivation rec {
     '';
 
     # Public domain + permissive (non-copyleft) licensing of some files.
-    license = "public domain";
+    license = stdenv.lib.licenses.publicDomain;
 
     homepage = http://people.csail.mit.edu/jaffer/SLIB;