The ebuild for links doesn't seem to use the USE flag for directfb support. It will be compiled in if the configure script detects it, so it works, but its not specifically enabled and listed as a dependency when the USE flag is on. It seems like this may be true for other packages as well. An emerge -UD world shouldn't leave binaries broken just because portage doesn't realize the dependencies.
from what i know the ebuild is using the fbcon flag. this flag "adds fb driver support for the console, via the kernel". so there is no need to add the directfb flag.
Created attachment 17179 [details, diff] patch for links from what i understand now is that fbcon will enable fb support. so there is no command to enable directfb support from command line. this patch will add directfb to depend and configure will hopefully enbale it automatically.
added to the ebuild, thanks Florian