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/java/classpath/default.nix

@@ -52,7 +52,7 @@ stdenv.mkDerivation rec {
     homepage = http://www.gnu.org/software/classpath/;
 
     # The exception makes it similar to LGPLv2+ AFAICS.
-    license = "GPLv2+ + exception";
+    license = stdenv.lib.licenses.gpl2ClasspathPlus;
 
     maintainers = [ ];
   };