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/libcec/default.nix

@@ -21,7 +21,7 @@ stdenv.mkDerivation {
     description = "USB CEC adapter communication library";
     homepage = "http://libcec.pulse-eight.com";
     repositories.git = "https://github.com/Pulse-Eight/libcec.git";
-    license = "GPLv2+";
+    license = stdenv.lib.licenses.gpl2Plus;
     platforms = platforms.linux;
     maintainers = [ maintainers.titanous ];
   };