Warn about missing -I paths

Fixes #121.  Note that we don't warn about missing $NIX_PATH entries
because it's intended that some may be missing (cf. the default
$NIX_PATH on NixOS, which includes paths like /etc/nixos/nixpkgs for
backward compatibility).

src/libexpr/eval.hh

@@ -120,7 +120,7 @@ public:
     EvalState();
     ~EvalState();
 
-    void addToSearchPath(const string & s);
+    void addToSearchPath(const string & s, bool warn = false);
 
     /* Parse a Nix expression from the specified file. */
     Expr * parseExprFromFile(const Path & path);