http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60470
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- It's also documented: http://gcc.gnu.org/install/prerequisites.html#TOC1 While it would be nice if it failed earlier, we don't want configure to fail for releases if flex isn't installed, because it isn't needed for releases, so it would be non-trivial to change this. If you're building a snapshot or building from the source repo then you are expected to be capable of installing an extra package or two.
