nameserv: add asynchronous resolving with POSIX threads

Run getaddrinfo()/gethostbyname() in separate thread to avoid blocking.
Only one resolving thread is running at one time, so this should work
also on systems where the functions are not thread-safe.

configure

@@ -112,6 +112,7 @@ 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
+  --disable-asyncdns     Disable asynchronous name resolving
   --disable-forcednsretry Don't retry on permanent DNS error
   --with-user=USER       Specify default chronyd user [root]
   --with-sendmail=PATH   Path to sendmail binary [/usr/lib/sendmail]
@@ -191,6 +192,7 @@ try_phc=0
 feat_pps=1
 try_setsched=0
 try_lockmem=0
+feat_asyncdns=1
 feat_forcednsretry=1
 default_user="root"
 mail_program="/usr/lib/sendmail"
@@ -267,6 +269,9 @@ do
     --disable-linuxcaps)
       feat_linuxcaps=0
     ;;
+    --disable-asyncdns)
+      feat_asyncdns=0
+    ;;
     --disable-forcednsretry)
       feat_forcednsretry=0
     ;;
@@ -419,6 +424,15 @@ then
   add_def HAVE_GETADDRINFO
 fi
 
+if [ $feat_asyncdns = "1" ] && \
+  test_code 'pthread' 'pthread.h' '-pthread' '' \
+    'return pthread_create(NULL, NULL, NULL, NULL);'
+then
+  add_def FEAT_ASYNCDNS
+  add_def USE_PTHREAD_ASYNCDNS
+  MYCFLAGS="$MYCFLAGS -pthread"
+fi
+
 timepps_h=""
 if [ $feat_pps = "1" ]; then
   if test_code '<sys/timepps.h>' 'sys/timepps.h' '' '' ''; then