app-admin/conky-1.9.0-r1 fails to compile with the "nvidia" USE flag enabled. It requires the header file /usr/include/NVCtrl/NVCtrl.h, which is only provided by the media-video/nvidia-settings package. A recent change on the conky-1.9.0-r1.ebuild changed the dependency for the "nvidia" USE flag from nvidia-settings to nvidia-drivers[tools], as it seems that nvidia-settings won't become stable in the future. However, the missing header is not provided by nvidia-drivers[tools]. This dependency on NVCtrl.h is a build time only dependency. Once conky is merged, nvidia-settings can be safely unmerged as long as nvidia-drivers[tools] is present.
Ugh, I've reported that one, see the "See Also" field... I think either nvidia-settings should be kept separately or upstream should be reported about the absence of that header file in nvidia-drivers, or perhaps conky doesn't necessarily need to depend upon it. Let's see...
+ 04 Dec 2012; Daniel Pielmeier <billie@gentoo.org> conky-1.9.0.ebuild, + conky-1.9.0-r1.ebuild, metadata.xml: + Revert previous commit as per bug #445926. I have reverted the commit and made conky depend on nvidia-settings again. Regarding the downgrade users have to wait until bug #443594 is fixed.