Add -lresolv on Darwin

[cptaffe]

On macOS 15.2, -lresolv is required to resolve symbols like _res_9_dn_expand:

; cd src/cmd/upas
; mk install
for i in common smtp alias fs ned q send marshal vf misc
do
	(cd $i; echo cd `pwd`';' mk $MKFLAGS install; mk $MKFLAGS install)
done
cd /Users/cptaffe/src/plan9port/src/cmd/upas/common; mk install
mk: 'install' is up to date
cd /Users/cptaffe/src/plan9port/src/cmd/upas/smtp; mk install
for i in smtp smtpd
do
	mk $MKFLAGS $i.install
done
9c -I../common -DSPOOL="/Users/cptaffe/src/plan9port/mail" rfc822.tab.c
9l -o o.smtp rfc822.tab.o mxdial.o smtp.o ../common/libcommon.a
Undefined symbols for architecture arm64:
  "_res_9_dn_expand", referenced from:
      _doquery in libndb.a[18](sysdnsquery.o)
      _doquery in libndb.a[18](sysdnsquery.o)
      _rrunpack in libndb.a[18](sysdnsquery.o)
  "_res_9_search", referenced from:
      _doquery in libndb.a[18](sysdnsquery.o)
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
mk: 9l -o o.smtp ...  : exit status=exit(1)
mk: for i in ...  : exit status=exit(1)
mk: for i in ...  : exit status=exit(1)

[bhuntsman]

Does this cause any unintended side effects on earlier versions of macOS?

[cptaffe]

I'm not sure, I don't have a machine running an older version. Is it possible to branch based on version here?

[cptaffe]

It looks like -lresolv has been in use as far back as OS X 10.6, to fix this exact linker error, see here.