The starting output of cmake will say "Not found." for USE="cg" because it's in /opt and the FindCg.cmake file lacks code/patch for it.
+# $Header: /var/cvsroot/gentoo-x86/media-gfx/nvidia-texture-tools/ChangeLog,v 1.3 2012/05/03 20:18:47 hasufell Exp $ + + 03 May 2012; Julian Ospald <hasufell@gentoo.org> + nvidia-texture-tools-2.0.8.ebuild, + +files/nvidia-texture-tools-2.0.8-cg.patch: + fix bug #414509