From a discussion on the forums, a mod suggested a feature request for portage. The thread is
basically, i had an emerge world fail, during the process of updating profile and gcc, after 7 hours, because the current version of evas required the use flag png. It would be nice if portage could check use flags and other environment factors (like required version of gcc/glibc, for example) for all packages about to be compile, to make sure everything is sane before starting the world emerge.
Until bug 2272 has been fixed, we use built_with_use from the eutils eclass. Portage already does non USE-based dependency checking.
*** This bug has been marked as a duplicate of 2272 ***