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/libjpeg/default.nix

@@ -1,20 +1,20 @@
-{ stdenv, fetchurl, static ? false }: 
+{ stdenv, fetchurl, static ? false }:
 
 with stdenv.lib;
 
 stdenv.mkDerivation {
   name = "libjpeg-8d";
-  
+
   src = fetchurl {
     url = http://www.ijg.org/files/jpegsrc.v8d.tar.gz;
     sha256 = "1cz0dy05mgxqdgjf52p54yxpyy95rgl30cnazdrfmw7hfca9n0h0";
   };
 
   configureFlags = optional static "--enable-static --disable-shared";
-  
+
   meta = {
     homepage = http://www.ijg.org/;
     description = "A library that implements the JPEG image file format";
-    license = "free";
+    license = stdenv.lib.licenses.free;
   };
 }