Add warning and fallback when auto intensity is 0

Occassionally the auto adjust doesn't find enough memory and the
intensity is detected too low and aligned to 0 with the compute units.
This patch fixes this situation by issuing a warning with a suggestion
to set environment vars and then ignoring the alignment to 0

Per several issues:

Principally:
https://github.com/fireice-uk/xmr-stak/issues/81

Related:
https://github.com/fireice-uk/xmr-stak/issues/490
https://github.com/fireice-uk/xmr-stak/issues/472