flex and bison files are not supported by cc-mode.
There is a flex-mode.el to be found here:
I have a copy, but I’m not sure about uploading it. There is no licence statement.
That is very unfortunate, because then we cannot distribute it. Does it have an email address or something? The upload script will only accept free software, too.
The link works now, so I guess this is no longer needed.
See also BisonMode.