This is kind of a shot in the dark, but try installing help2man and see if you get any farther?
(Based this on http://rainforce.org/flex/flex-2-5-10.notes -- the thought process in that file isn't entirely clear, so it's hard to be sure, but it might be worth a try.)
The Make output shows that it tries first to regenerate certain files which are normally not regenerated by users who just want to compile from the sources. One of those files is scan.c from scan.l which requires pre-existing flex and of course it fails.
This shouldn't be the case with downloaded tarballs. One guess is that file stamps are wrong. Try in a fresh directory
./configure
touch scan.c
make
If there is no scan.c file at all, then you have downloaded the wrong tarball or it is corrupted.
I downloaded a copy of flex-2.5.33. It contains both scan.l and scan.c.
-rw-r--r-- 500/500 141838 2006-02-20 20:45 flex-2.5.33/scan.c
-rw-r--r-- 500/500 20853 2006-02-14 13:28 flex-2.5.33/scan.l
It looks like you are missing the supplied scan.c.
From your make output it also looks like you are missing locale files or settings in your environment.
If you are going to be doing any other development on this machine with gnu based tools, you might consider installing autoconf and automake. It looks like the Makefile is forcing a reconfigure because those items are not present, which may be causing some unspecified issue.
Finally, you should consider having a second development machine where you can have a full development platform installed. That way you could compile/test there and see what 'normal' output should look like. Another alternative is to do your build on the development machine and install the results on your 'stripped down' target.