nicer date format and support for SOURCE_DATE_EPOCH in LXC_GENERATE_DATE

Using $(date) for LXC_GENERATE_DATE has various flaws:
* formating depends on the locale of the system we execute configure on
* the output is not really a date but more a timestamp

Let's use $(date --utc '+%Y-%m-%d') instead.

While at it, also support SOURCE_DATE_EPOCH [1] to make the build
reproducible

[1] https://reproducible-builds.org/specs/source-date-epoch/

Signed-off-by: Evgeni Golov <evgeni@debian.org>

configure.ac

@@ -545,7 +545,7 @@ AS_AC_EXPAND(LXC_DEFAULT_CONFIG, "$sysconfdir/lxc/default.conf")
 AS_AC_EXPAND(DATADIR, "$datadir")
 AS_AC_EXPAND(LOCALSTATEDIR, "$localstatedir")
 AS_AC_EXPAND(DOCDIR, "$docdir")
-AS_AC_EXPAND(LXC_GENERATE_DATE, "$(date)")
+AS_AC_EXPAND(LXC_GENERATE_DATE, "$(date --utc --date=@${SOURCE_DATE_EPOCH:-$(date +%s)} '+%Y-%m-%d')")
 AS_AC_EXPAND(LXCPATH, "$with_config_path")
 AS_AC_EXPAND(LXC_GLOBAL_CONF, "$with_global_conf")
 AS_AC_EXPAND(LXC_USERNIC_CONF, "$with_usernic_conf")