note the reason for auto-disabling GPU support in CMake

As the status message on unsuccesful CUDA detection has been muted, the
user is left clueless about why do we disable GPU support at configure
time. Extended the status message to note that this happens because no
compatible CUDA was found.

Some minor status message and code comment tweaks were also done.

Change-Id: Icfba7b56546997cd38952ceff2b7914c9f5b2d67
Merge "note the reason for auto-disabling GPU support in CMake" into release-4-6