Upgrading our minimum required flex version for 8.5

I'd like to return to the project I suggested here:
http://archives.postgresql.org/message-id/18653.1239741426@sss.pgh.pa.us
of getting rid of plpgsql's private lexer and having it use the core
lexer instead. This will require making the core lexer re-entrant,
which is not possible with our oldest supported version of flex
(2.5.4a). A look at the flex change history indicates that %reentrant
was added in 2.5.6 but nasty bugs were being fixed up to 2.5.30;
so the new minimum supported version would probably be 2.5.33
(2.5.31 was kinda broken for other reasons, and there was no 2.5.32).

Since 2.5.33 is now over three years old, this does not seem like an
onerous requirement, but I thought I'd better ask if anyone has an
objection?

regards, tom lane