clDevicesNum returned error, no GPUs usable
Bitcoin mining is principally an integer maths task, not a graphics task.  It just so happens that the hundreds of processors found on modern graphics cards can sometimes be suitable for parallel calculation of the bitcoin task, at upwards of a hundred times the best available rate from four cpu calculations.

See wiki reports on bitcoin hardware for comparison figures.

Unfortunately for you, Intel copied some parts of modern GPU designs into their system on a chip, but to save money they left out some transistors here and there which might not get used in direct X 11 graphics but would be necessary for parallel calculations for bitcoin.

So, you have on-chip graphics suitable for screen display and a few other specifically graphics tasks, but you do not have a general purpose massively parallel GPU for bitcoin calculations.

The error message "clDevicesNum returned error, no GPUs usable" appears on one of my systems which does have a suitable graphics card for bitcoin mining and does find openCL drivers for it when configured before compile.  I'm trying to find out what is incorrect in my configuration.