In a small pot, combine white vinegar and sugar and bring to a boil. Reduce to a rapid simmer and cook until mixture is syrupy, about 6 minutes. Remove from heat and stir in 3 garlic cloves, 2 teaspoons red-pepper flakes and 1/2 teaspoon coarse salt. Let cool to room temperature before using. (To store, refrigerate in an airtight container, up to 1 week).