* When using the build hook, distinguish between the stderr of the

hook script proper, and the stdout/stderr of the builder.  Only the
  latter should be saved in /nix/var/log/nix/drvs.
* Allow the verbosity to be set through an option.
* Added a flag --quiet to lower the verbosity level.

src/libutil/types.hh

@@ -63,7 +63,7 @@ typedef set<Path> PathSet;
 
  
 typedef enum { 
-    lvlError,
+    lvlError = 0,
     lvlInfo,
     lvlTalkative,
     lvlChatty,