I was indeed building from SVN, and not requiring flex and bison in configure makes sense now that I know you're pre-building the generated sources for the tarballs. I've tested r2008 and confirmed that configure still succeeds when flex and bison are missing (even with --enable-maintainer-mode) and that now it correctly points them at missing so that when make tries to use them you get a nice error message instead of the confusing ibConfLex.c not found errors I was getting before. Thanks! That was...
configure does not fail when flex is not found