Add PHC refclock driver

Implement a driver which allows using PTP hardware clock (PHC) as a
reference clock. It uses the PTP_SYS_OFFSET ioctl or clock_gettime()
to measure the offset between the PTP clock and the system clock. Ten
readings are made for every driver poll and the fastest one is returned.

As PHCs are typically kept in TAI instead of UTC, it's necessary to set
the TAI/UTC offset manually by the offset option. This could be improved
by obtaining the offset automatically from the right/UTC timezone.

configure

@@ -108,6 +108,7 @@ For better control, use the options below.
   --without-nss          Don't use NSS even if it is available
   --without-tomcrypt     Don't use libtomcrypt even if it is available
   --disable-ipv6         Disable IPv6 support
+  --disable-phc          Disable PHC support
   --disable-pps          Disable PPS API support
   --disable-rtc          Don't include RTC even on Linux
   --disable-linuxcaps    Disable Linux capabilities support
@@ -184,6 +185,8 @@ readline_lib=""
 readline_inc=""
 ncurses_lib=""
 feat_ipv6=1
+feat_phc=1
+try_phc=0
 feat_pps=1
 try_setsched=0
 try_lockmem=0
@@ -253,6 +256,9 @@ do
     --disable-ipv6)
       feat_ipv6=0
     ;;
+    --disable-phc)
+      feat_phc=0
+    ;;
     --disable-pps)
       feat_pps=0
     ;;
@@ -315,6 +321,7 @@ case $SYSTEM in
         try_rtc=1
         try_setsched=1
         try_lockmem=1
+        try_phc=1
         add_def LINUX
         echo "Configuring for " $SYSTEM
         if [ "${MACHINE}" = "alpha" ]; then
@@ -443,6 +450,13 @@ then
     add_def FEAT_RTC
 fi
 
+if [ $feat_phc = "1" ] && [ $try_phc = "1" ] && \
+  test_code '<linux/ptp_clock.h>' 'sys/ioctl.h linux/ptp_clock.h' '' '' \
+    'ioctl(1, PTP_CLOCK_GETCAPS, 0);'
+then
+    add_def FEAT_PHC
+fi
+
 if [ $try_setsched = "1" ] && \
   test_code \
     'sched_setscheduler()' \