Pour the reserved turkey pan juices into a glass measuring cup and skim off the fat. Set the roasting pan over 2 burners on moderately high heat. Add the wine and boil for 2 minutes, scraping up the browned bits from the bottom of the pan. Scrape the contents of the pan into a strainer set over the turkey broth and press on the vegetables. Bring the broth to a boil. Mix the cornstarch with the remaining 1/2 cup of water until smooth, then whisk this slurry into the boiling broth. Reduce the heat to low and simmer until lightly thickened, about 2 minutes. Season with pepper, pour into a gravy boat and serve.