sys_macosx: add support for ntp_adjtime() on macOS 10.13+

macOS 10.13 will implement the ntp_adjtime() system call, allowing
better control over the system clock than is possible with the existing
adjtime() system call. chronyd will support both the older and newer
calls, enabling binary code to run without recompilation on macOS 10.9
through macOS 10.13.

Early releases of macOS 10.13 have a very buggy adjtime() call. The
macOS driver tests adjtime() to see if the bug has been fixed. If the
bug persists then the timex driver is invoked otherwise the netbsd
driver.

configure

@@ -428,6 +428,15 @@ case $OPERATINGSYSTEM in
           add_def FEAT_PRIVDROP
           priv_ops="ADJUSTTIME SETTIME BINDSOCKET"
         fi
+        major=`echo $VERSION | cut -d. -f1`
+        # ntp_adjtime is not available in macOS 10.12 (Darwin 16.x.x) and earlier
+        if [ $major -gt "16" ]; then
+          add_def HAVE_MACOS_SYS_TIMEX
+          EXTRA_OBJECTS="$EXTRA_OBJECTS sys_generic.o sys_netbsd.o sys_timex.o"
+          if [ $feat_droproot = "1" ]; then
+            priv_ops="$priv_ops ADJUSTTIMEX"
+          fi
+        fi
         echo "Configuring for macOS (" $SYSTEM "macOS version" $VERSION ")"
     ;;
     SunOS)