gpuowl

At Mihai's suggestion, use a portable method to sleep N microseconds.

Solved the nVidia finish causing a CPU busy wait. The queue contains at most 500 kernels, a marker, and 500 more kernels. When prpll tries to add another kernel to the queue, we loop checking if the marker has been reached else performing a lengthy sleep (knowing there are 500 kernels to execute after the marker).