Replace build_unique with make_unique

This was a rarely used convenience function from when we did not yet
have c++17 support.

src/Var.cc

@@ -510,7 +510,7 @@ void end_func(Stmt* body)
 
 std::unique_ptr<function_ingredients> gather_function_ingredients(Scope* scope, Stmt* body)
 	{
-	auto ingredients = build_unique<function_ingredients>();
+	auto ingredients = make_unique<function_ingredients>();
 
 	ingredients->frame_size = scope->Length();
 	ingredients->inits = scope->GetInits();