Add the ability to sample GPU_POWER sensors.  Because
the sensors have a high latency we read them from a background
thread which means we've added the requirement for pthreads.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Kai Wasserbäch <kai@dev.carbon-project.org>

(v2) Cleaned up CMake around pthreads
(v3) Update readme, factor out some deletions, cleanup switch statement