licenses: comment about two versions of MIT

I decided to follow spdx.org and not to differentiate those two.
Packages would often have the wrong version anyway.

lib/licenses.nix

@@ -194,6 +194,8 @@ rec {
     fullName = "Lucent Public License v1.02";
   };
 
+  # spdx.org does not (yet) differentiate between the X11 and Expat versions
+  # for details see http://en.wikipedia.org/wiki/MIT_License#Various_versions
   mit = spdx {
     shortName = "MIT";
     fullName = "MIT License";

pkgs/applications/audio/pianobar/default.nix

@@ -21,6 +21,6 @@ stdenv.mkDerivation rec {
     description = "A console front-end for Pandora.com";
     homepage = "http://6xq.net/projects/pianobar/";
     platforms = stdenv.lib.platforms.linux;
-    license = stdenv.lib.licenses.mit;
+    license = stdenv.lib.licenses.mit; # expat version
   };
 }

pkgs/development/libraries/expat/default.nix

@@ -8,9 +8,10 @@ stdenv.mkDerivation rec {
     sha256 = "11pblz61zyxh68s5pdcbhc30ha1b2vfjd83aiwfg4vc15x3hadw2";
   };
 
-  meta = {
+  meta = with stdenv.lib; {
     homepage = http://www.libexpat.org/;
     description = "A stream-oriented XML parser library written in C";
-    platforms = stdenv.lib.platforms.all;
+    platforms = platforms.all;
+    license = licenses.mit; # expat version
   };
 }