meson: initialize time-epoch to reproducible builds compatible value

Debian Policy encourages to preserve timestamps whenever possible in the
tarballs, thus stable release updates of systemd usually do not bump NEWS file
timestamp. And thus time-epoch remains the same for the lifetime of a release.

It would be better, if each new stable release rebuild of systemd would bump
the time epoch a bit. But at the same time remain
reproducible. SOURCE_DATE_EPOCH is an environmnet variable defined for this
purpose. Thus if available, prefer that, instead of the NEWS file modification
time.

For example, on Debian/Ubuntu under the reproducible builds the
SOURCE_DATE_EPOCH is set to the timestamp from the packaging metadata, thus it
is incremented on every new stable release update, whilst preserving
reproducible builds capability.

Reference: https://reproducible-builds.org/docs/timestamps/

TODO

@@ -511,9 +511,6 @@ Features:
 
 * support projid-based quota in machinectl for containers
 
-* maybe use SOURCE_DATE_EPOCH (i.e. the env var the reproducible builds folks
-  introduced) as the RTC epoch, instead of the mtime of NEWS.
-
 * add a way to lock down cgroup migration: a boolean, which when set for a unit
   makes sure the processes in it can never migrate out of it
 

meson.build

@@ -669,8 +669,13 @@ conf.set_quoted('DEFAULT_NET_NAMING_SCHEME', default_net_naming_scheme)
 
 time_epoch = get_option('time-epoch')
 if time_epoch == -1
-        NEWS = files('NEWS')
-        time_epoch = run_command(stat, '-c', '%Y', NEWS).stdout().to_int()
+        source_date_epoch = run_command('sh', ['-c', 'echo "$SOURCE_DATE_EPOCH"']).stdout().strip()
+        if source_date_epoch != ''
+                time_epoch = source_date_epoch.to_int()
+        else
+                NEWS = files('NEWS')
+                time_epoch = run_command(stat, '-c', '%Y', NEWS).stdout().to_int()
+        endif
 endif
 conf.set('TIME_EPOCH', time_epoch)