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/libcaca/default.nix

@@ -2,19 +2,19 @@
 
 stdenv.mkDerivation rec {
   name = "libcaca-0.99.beta19";
-  
+
   src = fetchurl {
     url = "http://caca.zoy.org/files/libcaca/${name}.tar.gz";
     sha256 = "1x3j6yfyxl52adgnabycr0n38j9hx2j74la0hz0n8cnh9ry4d2qj";
   };
-  
+
   configureFlags = "--disable-x11 --disable-imlib2 --disable-doc";
-  
+
   propagatedBuildInputs = [ncurses];
 
   meta = {
     homepage = http://libcaca.zoy.org/;
     description = "A graphics library that outputs text instead of pixels";
-    license = "WTFPL"; # http://sam.zoy.org/wtfpl/
+    license = stdenv.lib.licenses.wtfpl;
   };
 }