Ninja: Refactor generation of 'restat' on custom commands

Move generation of 'restat = 1' from the CUSTOM_COMMAND rule to every
build statement using it.  This will allow future selection of this
option on a per-custom-command basis.

Source/cmGlobalNinjaGenerator.cxx

@@ -275,7 +275,7 @@ void cmGlobalNinjaGenerator::AddCustomCommandRule()
                 /*deptype*/ "",
                 /*rspfile*/ "",
                 /*rspcontent*/ "",
-                /*restat*/ "1",
+                /*restat*/ "", // bound on each build statement as needed
                 /*generator*/ false);
 }
 
@@ -300,6 +300,7 @@ cmGlobalNinjaGenerator::WriteCustomCommandBuild(const std::string& command,
   cmNinjaVars vars;
   vars["COMMAND"] = cmd;
   vars["DESC"] = EncodeLiteral(description);
+  vars["restat"] = "1";
   if (uses_terminal && SupportsConsolePool())
     {
     vars["pool"] = "console";