Both mail-filter/libmilter and mail-mta/sendmail installs the libmilter library as static one. Therefore, the "milter? ..." atom should be removed from RDEPEND and this atom should be added to DEPEND: milter? ( || ( mail-filter/libmilter mail-mta/sendmail ) ) Note the reverse order, which will have no effect on sendmail users but will make life a little easier for those who use postfix.
Fixed in CVS, thanks. (I bet you expected this bug to rot in peace for few more months. :) )