build-sys: warn if people don't change the default NTP servers when building systemd

Also, explain the situation in the docs.

Relates to #437

DISTRO_PORTING

@@ -14,6 +14,7 @@ HOWTO:
             --with-kbd-loadkeys=
             --with-kbd-setfont=
             --with-tty-gid=
+            --with-ntp-servers=
 
         2) Try it out. Play around (as an ordinary user) with
         '/usr/lib/systemd/systemd --test --system' for a test run
@@ -21,6 +22,24 @@ HOWTO:
         print the initial transaction it would execute during boot-up.
         This will also inform you about ordering loops and suchlike
 
+NTP POOL:
+
+        By default, timesyncd uses the Google NTP servers
+        time[1-4].google.com. They serve time that is not standards
+        compliant, and can be up to .5s off. Google does not
+        officially support these servers for the broader
+        audience. Distributions and vendors really should not ship
+        OSes or devices with these NTP servers configured. Instead,
+        please register your own vendor pool at ntp.org and make it
+        the built-in default by passing --with-ntp-servers= to
+        configure. Registering vendor pools is free:
+
+        http://www.pool.ntp.org/en/vendors.html
+
+        Again, if you ship your software or device with the default
+        NTP servers, then you will get served wrong time, and will
+        rely on services that might not be supported for long.
+
 CONTRIBUTING UPSTREAM:
 
         We generally do no longer accept distribution-specific

README

@@ -261,6 +261,11 @@ WARNINGS:
         false positives will be triggered by code which violates
         some rules but is actually safe.
 
+        Currently, systemd-timesyncd defaults to use the Google NTP
+        servers if not specified otherwise at configure time. You
+        really should not ship an OS or device with this default
+        setting. See DISTRO_PORTING for details.
+
 ENGINEERING AND CONSULTING SERVICES:
         ENDOCODE <https://endocode.com/> offers professional
         engineering and consulting services for systemd. Please

configure.ac

@@ -1009,7 +1009,8 @@ AC_ARG_WITH(ntp-servers,
         AS_HELP_STRING([--with-ntp-servers=NTPSERVERS],
                 [Space-separated list of default NTP servers]),
         [NTP_SERVERS="$withval"],
-        [NTP_SERVERS="time1.google.com time2.google.com time3.google.com time4.google.com"])
+        [NTP_SERVERS="time1.google.com time2.google.com time3.google.com time4.google.com"
+        AC_MSG_WARN([*** Using Google NTP servers. Please do not ship OSes or devices with these default settings. See DISTRO_PORTING for details!])])
 
 AC_DEFINE_UNQUOTED(NTP_SERVERS, ["$NTP_SERVERS"], [Default NTP Servers])
 AC_SUBST(NTP_SERVERS)