Turn some license strings into lib.licenses values

pkgs/development/libraries/vmime/default.nix

@@ -13,7 +13,7 @@ stdenv.mkDerivation {
   meta = {
     homepage = http://www.vmime.org/;
     description = "Free mail library for C++";
-    license = "GPLv3";
+    license = stdenv.lib.licenses.gpl3;
     maintainers = with stdenv.lib.maintainers; [viric];
     platforms = with stdenv.lib.platforms; linux;
   };