ntp: add support for software timestamping on Linux

Enable SCM_TIMESTAMPING control messages and the socket's error queue in
order to receive our transmitted packets with a more accurate transmit
timestamp. Add a new file for Linux-specific NTP I/O and implement
processing of these messages there.

configure

@@ -101,6 +101,7 @@ For better control, use the options below.
   --disable-asyncdns     Disable asynchronous name resolving
   --disable-forcednsretry Don't retry on permanent DNS error
   --without-clock-gettime Don't use clock_gettime() even if it is available
+  --disable-timestamping Disable support for SW timestamping
   --enable-ntp-signd     Enable support for MS-SNTP authentication in Samba
   --with-ntp-era=SECONDS Specify earliest assumed NTP time in seconds
                          since 1970-01-01 [50*365 days ago]
@@ -219,6 +220,8 @@ try_lockmem=0
 feat_asyncdns=1
 feat_forcednsretry=1
 try_clock_gettime=1
+feat_timestamping=1
+try_timestamping=0
 feat_ntp_signd=0
 ntp_era_split=""
 default_user="root"
@@ -329,6 +332,9 @@ do
     --without-clock-gettime)
       try_clock_gettime=0
     ;;
+    --disable-timestamping)
+      feat_timestamping=0
+    ;;
     --enable-ntp-signd)
       feat_ntp_signd=1
     ;;
@@ -387,6 +393,7 @@ case $OPERATINGSYSTEM in
         [ $try_libcap != "0" ] && try_libcap=1
         try_rtc=1
         [ $try_seccomp != "0" ] && try_seccomp=1
+        try_timestamping=1
         try_setsched=1
         try_lockmem=1
         try_phc=1
@@ -459,6 +466,7 @@ if [ $feat_ntp = "1" ]; then
   fi
 else
   feat_asyncdns=0
+  feat_timestamping=0
 fi
 
 if [ "$feat_cmdmon" = "1" ] || [ $feat_ntp = "1" ]; then
@@ -633,6 +641,19 @@ else
   fi
 fi
 
+if [ $feat_timestamping = "1" ] && [ $try_timestamping = "1" ] &&
+  test_code 'SW timestamping' 'sys/types.h sys/socket.h linux/net_tstamp.h
+                               linux/errqueue.h' '' '' '
+    int val = SOF_TIMESTAMPING_SOFTWARE | SOF_TIMESTAMPING_RX_SOFTWARE |
+              SOF_TIMESTAMPING_OPT_CMSG;
+    return sizeof (struct scm_timestamping) + SCM_TSTAMP_SND +
+           setsockopt(0, SOL_SOCKET, SO_SELECT_ERR_QUEUE + SO_TIMESTAMPING,
+                      &val, sizeof (val));'
+then
+  add_def HAVE_LINUX_TIMESTAMPING
+  EXTRA_OBJECTS="$EXTRA_OBJECTS ntp_io_linux.o"
+fi
+
 timepps_h=""
 if [ $feat_refclock = "1" ] && [ $feat_pps = "1" ]; then
   if test_code '<sys/timepps.h>' 'sys/timepps.h' '' '' ''; then