Patch submitted upstream at https://bugzilla.mindrot.org/show_bug.cgi?id=2697

diff -u -r ../openssh-7.5p1/configure.ac ./configure.ac
--- ../openssh-7.5p1/configure.ac	2017-03-20 03:39:27.000000000 +0100
+++ ./configure.ac	2017-03-23 11:18:49.800171062 +0100
@@ -1486,6 +1486,7 @@
 		else
 			LIBS="$LIBS `$LDNSCONFIG --libs`"
 			CPPFLAGS="$CPPFLAGS `$LDNSCONFIG --cflags`"
+			ldns=yes
 		fi
 	elif test "x$withval" != "xno" ; then
 			CPPFLAGS="$CPPFLAGS -I${withval}/include"