Question about binpac

Hello and greetings.

I have bro-1.4 source. In the aux/binpac/src directory, there is the file that is generated
using flex as the comment says.
Is this pre-generated rather than at make time?
Since I am not familiar with the flex, could someone shed some light
on how that is created?
I could not find if there was a line in the makefile for that.

Any help is much appreciated.