If you don't have both flex and bison installed, both libpcap and
bro will use the system lex/yacc. The system lex/yacc don't support
multiple parsers in the same executable.
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.