I was working on a smoke sim in 2.78c and when I tried to render, I got nothing. I noticed my device was set to GPU Rendering (with no GPU in my laptop) so I set it back to CPU. Nothing. I checked User Preferences, and it shows None in Cycles Compute Device. I reloaded the factory settings, but it still gives me nothing for compute devices. I'm pretty sure I still have a CPU...? Anyone know what might be causing this?
With the reference to this BUG report, if you have an AMD card, it looks like it is not supported anymore if your card's architecture version is less than 2.0
If you still want to enable it, on the bug report page is a link to a workaround. I'm not adding the workaround details here as it has some warning and will be better if you read from the original page.