[C++] Add .KATI_RESTAT builtin target

This is an experimental kati-specific expansion for GNU make.
Targets specified by this will have "restat = 1" in generated
ninja files.

Even with this change, kati should be still compatible with
GNU make. GNU make will ignore this but this only means GNU
make will do some extra unnecessary builds. This different
should not change the final output as long as .KATI_RESTAT is
used appropriately.

TODO: Implement the same feature in exec.cc and add a test.
diff --git a/dep.h b/dep.h
index 961ca0a..7fe8439 100644
--- a/dep.h
+++ b/dep.h
@@ -30,7 +30,7 @@
 class Vars;
 
 struct DepNode {
-  DepNode(Symbol output, bool is_phony);
+  DepNode(Symbol output, bool is_phony, bool is_restat);
 
   Symbol output;
   vector<Value*> cmds;
@@ -38,8 +38,9 @@
   vector<DepNode*> order_onlys;
   vector<DepNode*> parents;
   bool has_rule;
-  bool is_phony;
   bool is_default_target;
+  bool is_phony;
+  bool is_restat;
   vector<Symbol> actual_inputs;
   Vars* rule_vars;
   Symbol output_pattern;