Add configure option to force retry on DNS failure

This is apparently needed on system which keep nameservers specified
in /etc/resolv.conf even when there is no network connection. Should be
used with care as invalid names will be retried forever.

configure

@@ -102,6 +102,7 @@ For better control, use the options below.
   --disable-pps          Disable PPS API support
   --disable-rtc          Don't include RTC even on Linux
   --disable-linuxcaps    Disable Linux capabilities support
+  --enable-forcednsretry Force retry on DNS failure
 
 Fine tuning of the installation directories:
   --sysconfdir=DIR       chrony.conf location [/etc]
@@ -162,6 +163,7 @@ feat_ipv6=1
 feat_pps=1
 try_setsched=0
 try_lockmem=0
+feat_forcednsretry=0
 
 for option
 do
@@ -226,6 +228,9 @@ do
     --disable-linuxcaps)
       feat_linuxcaps=0
     ;;
+    --enable-forcednsretry)
+      feat_forcednsretry=1
+    ;;
     --host-system=* )
       OPERATINGSYSTEM=`echo $option | sed -e 's/^.*=//;'`
     ;;
@@ -399,6 +404,11 @@ then
   SYSDEFS="${SYSDEFS} -DHAVE_MLOCKALL"
 fi
 
+if [ $feat_forcednsretry = "1" ]
+then
+    EXTRA_DEFS="$EXTRA_DEFS -DFORCE_DNSRETRY=1"
+fi
+
 READLINE_COMPILE=""
 READLINE_LINK=""
 if [ $feat_readline = "1" ]; then