Expand PeekVar to KATI_(deprecated|obsolete)_var

So that marking a variable as deprecated or obsolete does not cause the
variable to be inserted into the used environment table.
diff --git a/eval.h b/eval.h
index 1dff4b7..e588f7e 100644
--- a/eval.h
+++ b/eval.h
@@ -47,6 +47,9 @@
   // For target specific variables.
   Var* LookupVarInCurrentScope(Symbol name);
 
+  // Equivalent to LookupVar, but doesn't mark as used.
+  Var* PeekVar(Symbol name);
+
   string EvalVar(Symbol name);
 
   const Loc& loc() const { return loc_; }