In a large saucepan, bring stock to a low boil over medium heat; cook until reduced by half, about 30 minutes.
Meanwhile, place porcini in a small bowl, and cover with boiling water. Let soak until porcini are soft, about 10 minutes. Remove porcini, and gently squeeze out excess water; finely chop. Set aside.
Heat butter in a medium saucepan over medium-high heat until foamy. Add white mushrooms, and season with salt and pepper. Add reserved porcini. Cook, stirring, until white mushrooms are soft, about 10 minutes.
Sprinkle flour over mushroom mixture, and cook until flour starts to brown, a few minutes more. Add wine; deglaze pan, scraping up browned bits on bottom and sides with a wooden spoon.
Stir stock into mushroom mixture. Bring to a gentle simmer over very low heat, and cook until thickened slightly, about 30 minutes. If gravy thickens too much, thin with a little water or stock. Season with salt and pepper; serve hot.
The gravy can be prepared up to one day ahead, then reheated over low heat.