I've found some conflicting information.
It seems OpenCL 1.2 is only possible on Ubuntu 14.04 (and you must downgrade the Linux kernel to 3.19):
Also, some AMD support folks recommend installing APP SDK 3.0, but other AMD support folks say that this is deprecated / incompatible with the newer amgdpu-pro drivers:
I'm trying various combinations, but I think the motherboard I'm using won't work because it provides an Intel graphics chip (i915) and doesn't provide any way to disable it - and there seems to be a weird conflict with X11/RX 580/i915. I tried blacklisting the i915, but X11 still seems to find it (via fbdev iirc).
So I've ordered a new motherboard and Ryzen CPU. When I get it, should I be able to use OpenCL += 1.2 with Ubuntu 16.04 or should I downgrade to 14.04? (and downgrade the kernel to 3.19?)