Despite the naysayers, this is not a stupid statement, nor is it impossible.
There are now two large infrastructure projects planned or underway on the Canadian border. Both projects will benefit both countries, perhaps equally. Both projects, if built, will be financed totally by the Canadian government or investors in a Canadian company. US taxpayers will not pay a dime.
Even I can think of two ways to "make Mexico pay". One is a tax on remittances to Mexico. Another is a tax on Mexican citizens entering the US legally at ports of entry. I'm sure Trump has thought of this.
He is not so dumb. Another thread on FR claims that he has snagged the top political consultant in Iowa. He is second in New Hampshire, where his bombast might not play well. If he wins those two, Katy bar the door.
Mexico building the wall reminds me of the scene from the Godfather:
Senator Pat Geary: I want your answer and the money by noon tomorrow. And one more thing. Don’t you contact me again, ever. From now on, you deal with Turnbull.
Michael Corleone: Senator? You can have my answer now, if you like. My final offer is this: nothing. Not even the fee for the gaming license, which I would appreciate if you would put up personally.