During an attempt to add a 915resolution patch to the grub-1.96 ebuild, required for the eeePC. I found it required ruby to work properly. Without ruby, the common.mk won't get generated from common.rmk by the genmk.rb script. By default this behaviour is not exposed since common.mk is already present. So it requires the addition of DEPEND=dev-lang/ruby
the .mk files are distributed. ruby is only needed when those files get regenerated (which doesnt happen in the ebuild).