GN Mac build work

tools/gn/args.cc

Index: tools/gn/args.cc
diff --git a/tools/gn/args.cc b/tools/gn/args.cc
index 2eead427b88349186e2fea5274c2d82d36186104..2d9aa6e81d0cc670a8a79dd14ffae49c33f83eea 100644
--- a/tools/gn/args.cc
+++ b/tools/gn/args.cc
@@ -135,9 +135,12 @@ bool Args::DeclareArgs(const Scope::KeyValueMap& args,
     }
 
     // Only set on the current scope to the new value if it hasn't been already
-    // set.
-    if (!scope_to_set->GetValue(i->first))
+    // set. Mark the variable used so the build script can override it in
+    // certain cases without getting unused value errors.
+    if (!scope_to_set->GetValue(i->first)) {
       scope_to_set->SetValue(i->first, i->second, i->second.origin());
+      scope_to_set->MarkUsed(i->first);
+    }
   }
 
   return true;