bind9 build sometimes fails when an older bind9 is installed

Summary

bind9's build finds installed (older) versions of bind9 which sometimes causes build failures (ie 9.18.0 fails when 9.16.26 is installed)

BIND version used

9.18.0

Steps to reproduce

Build + install bind9 9.16.26 into some place the build searches. Try to build bind 9.18.0

What is the current bug behavior?

link fails w/ undefined symbols error

What is the expected correct behavior?

build succeeds

Relevant configuration files

Relevant logs and/or screenshots

:info:build   CC       cfg_test-cfg_test.o
:info:build   CCLD     cfg_test
:info:build Undefined symbols for architecture x86_64:
:info:build   "_isc_mem_create", referenced from:
:info:build       _main in cfg_test-cfg_test.o
:info:build   "_isc_mem_destroy", referenced from:
:info:build       _main in cfg_test-cfg_test.o
:info:build ld: symbol(s) not found for architecture x86_64
:info:build clang: error: linker command failed with exit code 1 (use -v to see invocation)

Possible fixes

I'll attach a patch that fixes the issue (used by MacPorts)