configure: add option --without-clock-gettime

clock_gettime() will be ignored even if it is present

configure

@@ -100,6 +100,7 @@ For better control, use the options below.
   --without-seccomp      Don't use seccomp even if it is available
   --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
   --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]
@@ -215,6 +216,7 @@ try_setsched=0
 try_lockmem=0
 feat_asyncdns=1
 feat_forcednsretry=1
+try_clock_gettime=1
 feat_ntp_signd=0
 ntp_era_split=""
 default_user="root"
@@ -320,6 +322,9 @@ do
     --disable-forcednsretry)
       feat_forcednsretry=0
     ;;
+    --without-clock-gettime)
+      try_clock_gettime=0
+    ;;
     --enable-ntp-signd)
       feat_ntp_signd=1
     ;;
@@ -569,18 +574,18 @@ then
   fi
 fi
 
-if test_code 'clock_gettime()' 'time.h' '' '' \
+if [ $try_clock_gettime = "1" ]; then
-  'clock_gettime(CLOCK_REALTIME, NULL);'
+  if test_code 'clock_gettime()' 'time.h' '' '' \
-then
-  add_def HAVE_CLOCK_GETTIME
-else
-  if test_code 'clock_gettime() in -lrt' 'time.h' '' '-lrt' \
     'clock_gettime(CLOCK_REALTIME, NULL);'
   then
     add_def HAVE_CLOCK_GETTIME
-    EXTRA_LIBS="$EXTRA_LIBS -lrt"
   else
-    try_phc=0
+    if test_code 'clock_gettime() in -lrt' 'time.h' '' '-lrt' \
+      'clock_gettime(CLOCK_REALTIME, NULL);'
+    then
+      add_def HAVE_CLOCK_GETTIME
+      EXTRA_LIBS="$EXTRA_LIBS -lrt"
+    fi
   fi
 fi
 
@@ -688,6 +693,7 @@ then
 fi
 
 if [ $feat_refclock = "1" ] && [ $feat_phc = "1" ] && [ $try_phc = "1" ] && \
+  grep '#define HAVE_CLOCK_GETTIME' config.h > /dev/null && \
   test_code '<linux/ptp_clock.h>' 'sys/ioctl.h linux/ptp_clock.h' '' '' \
     'ioctl(1, PTP_CLOCK_GETCAPS, 0);'
 then