dns/bind9*: fix WITH_DEBUG build

The READLINE_CFLAGS="-L${LOCALBASE}/lib" env came with bind 9.18 with no
explanation. With 9.20 it actually breaks the developer build:

cc: error: argument unused during compilation: '-L/usr/local/lib' [-Werror,-Wunused-command-line-argument]

The --enable-symtable configure option is no longer supported by bind and
just produces a warning.

Reviewed by: mat
Differential Revision: https://reviews.freebsd.org/D46540