build: symbol ‘bfd_auth_type’ is used, but is not defined as a token and has no rules

[Zhang Maiyun via Bird-users]

Hello,

I was trying to build the latest commit and encountered this error:
nest/config.Y:615.19-31: error: symbol ‘bfd_auth_type’ is used, but is not 
defined as a token and has no rules

It looks like commit e6dbde688 introduced a dependency on the type 
`bfd_auth_type` into `nest/config.Y`.
However, if protocol bfd is not enabled (e.g. because of disabling pthreads), 
`proto/bfd/config.Y` won’t get included in the parser and thus `bfd_auth_type` 
is undefined.

Perhaps this should be fixed?

Thanks!
Maiyun Zhang (he/him)