This is a very old problem, but IMHO should be fixed in portage in possible way! Details e.g. here: http://bugs.gentoo.org/show_bug.cgi?id=243004 There are many scenarious where this can happen, even if I set the -avahi useflag kdelibs3 pulls in avahi support (whereas it shouldnt), perhaps this could be changed. At least with the -avahi useflag the user shouldn't run into trouble
*** This bug has been marked as a duplicate of bug 243004 ***