The libffi useflag recently added for gcc should be documented in either /usr/portage/profiles/use.desc or /usr/portage/profiles/use.local.desc, as it is, tools like flagedit reports: warning : 'libffi' seems to be a wrong flag, I hope you know what you're doing every time I edit any useflag after turning on the libffi useflag. See also bug #71127 (gcc needs a libffi USE flag).
added to cvs http://sources.gentoo.org/profiles/use.local.desc?r1=1.3366&r2=1.3367