Anyone seen this problem before ?

I wonder if bro's configure script should insist on flex/bison,
since it knows that it'll be linked with libpcap which already has
a parser, and if both are compiled with the system lex/yacc you will
get errors like this.

Yep, you're right, it should. The fallback to lex/yacc is vestigial.
If someone wants to contribute the right configure tweak to enforce
flex/bison, that would be cool ....