<nods>
Sorry, most of you just don't get it - there's only one answer. Some of the above are very creative (and some are worth the laugh intended), but not very realistic.
Because the Trade Depot is the key, for the caravan. Every(?) dwarf caravan carries blocks of some sort - there's everything you need. Blocks, barrels, empty or full of some drink or food, plus at least two cloth/leather bins. Possibly an axe and a pick, but if you get an anvil you're making your own anyway at that point.
So the challenge is twofold - both lasting until mid-Autumn when the caravan arrives, and having the several thousand of dwarfbucks worth of goods to trade for the goods, in case only a steel anvil shows up.
From test-runs I've done (early experiments, ignoring all else while playing with other game factors), dwarfs tend to start to crack by mid- or late-summer if they're sleeping outside and in the dirt, with nothing produced to distract them, even with a mountain of alcohol and food lying next to them.
Without that one pick to get them inside at least, chances are most won't last to the arrival of the caravan. What's more, that pick also gets the blocks to build any and every workshop - some chairs and tables and statues, some armor and bags, some bone crossbows, toss in some smoothed walls for your dining hall - your dwarfs may have the DT's and drink from murky pools for a few seasons, but you're home free after that.
And that's THE dealbreaker.
(I'll grant that with an axe you could construct actual log-roofed buildings to get them "inside" - but if no one mentioned that, I'm guessing they missed this little detail entirely. And while wood chairs/tables/statues/trade items as impressive, and valuable, as common stone, you have nothing but time for exploratory mining to find something better.)