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/asio/default.nix

@@ -2,7 +2,7 @@
 
 stdenv.mkDerivation rec {
   name = "asio-1.10.4";
-  
+
   src = fetchurl {
     url = "mirror://sourceforge/asio/${name}.tar.bz2";
     sha256 = "0jminwr84wphwpph7j820cql7cbaqlj2zv4gfjk2imazpmlhsfri";
@@ -14,7 +14,7 @@ stdenv.mkDerivation rec {
   meta = {
     homepage = http://asio.sourceforge.net/;
     description = "Cross-platform C++ library for network and low-level I/O programming";
-    license = "boost";
+    license = stdenv.lib.licenses.boost;
   };
 
 }