What does this configure warning mean?

When I run ./configure, I get

(...warning..)
(If you are using the official distribution of PostgreSQL then you do
not need to worry about this because the Flex output is pre-generated.)

Well, I am am using the official distribution of PostreSQL and the
output is not pre-generated. Clearly this message only comes when you
build Postgres from source, in which case the output cannot be
pre-generated (can it?). 

It seems like a very strange warning to me.

-- 
 Simon Riggs           www.2ndQuadrant.com