From what I can tell, only consistent heat exposure can develop heat tolerance. By July each year I've acclimated to the summer and can cycle, hike, etc. in 100+ temperatures, whereas the earlier season heatwaves knock me flat. The flip side is when camping at 10k feet, cold can feel COLD.
I suspect most healthy people can indeed develop heat tolerance by building up to it. If you're lucky enough to live on the foggy coast you're going to get hammered backpacking up a sunny mountain in 95-degree heat. But if you live inland, try to get out in the heat regularly enough that your body can get used to it. Regardless, it's good to hike early and late when its hot, rest in the middle of the day, and work hard at staying hydrated.