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]
Indeed, -L was never a compiler flag.
The --enable-symtable configure option is no longer supported by
bind and just produces a warning.