Sweat...
Work harder.What did people do before smart phone/pocket computers?
Work harder.
Get tougher.
Invent smart phones and pocket computers.
Seneca, the stoic philosopher, mocked the "skinny youths" who ate snow to keep cool rather than simply bearing the heat like a real Roman ought to.
Here's an interesting related question, what effect has A/C had on our country?
For one thing, sun belt cities that had been unbearable during the summer heat became more attractive places to live. A/C thus facilitated a long term shift of the U.S. population to the south.
And, lest we forget, LOTS of people around the world STILL make due without A/C (esp in Africa and southern Asia).