You could try melted sugar (caramelized sugar), which acts like hot glue. Just melt about 1 cup of sugar in a skillet over low to medium-low on the stove. Wait for it to melt, constantly stirring, until it just starts turning a pale golden color. Dip the edges of your gingerbread into the sugar, press them together, and hold for a few seconds. The sugar will harden as it cools, making for a great "glue." Keep it over low heat while assembling the house so the sugar doesn't harden. If you don't plan on eating it, you could even use hot glue to assemble the house.