bison (and flex) are only needed when building from a CVS checkout of the PG sources. The release tarballs already include the files that are generated by flex and bison. The error message that configure displays should probably make clear that it can be ignored if you're building from a tarball - I'd actually update the OACS documentation to note that the configure error is harmless.
http://developer.postgresql.org/cvsweb.cgi/pgsql-server/README.CVS?rev=1.3
