Merge 'staging' into closure-size

- there were many easy merge conflicts
- cc-wrapper needed nontrivial changes

Many other problems might've been created by interaction of the branches,
but stdenv and a few other packages build fine now.

pkgs/development/libraries/libjpeg/default.nix

@@ -17,6 +17,6 @@ stdenv.mkDerivation {
   meta = {
     homepage = http://www.ijg.org/;
     description = "A library that implements the JPEG image file format";
-    license = "free";
+    license = stdenv.lib.licenses.free;
   };
 }