Allow to override build date with SOURCE_DATE_EPOCH

to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

This date call works with different variants of date (GNU, FreeBSD, ...).

This patch was done while working on reproducible builds for openSUSE.

proj_mgmt/DATE_cmd.sh

@@ -13,5 +13,8 @@ if [ $osType = "Darwin" ]; then
   fi
 fi
 
-$my_cmd "$arg"
+SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$($my_cmd +%s)}"
+$my_cmd -d "@$SOURCE_DATE_EPOCH" "$arg" 2>/dev/null ||
+  $my_cmd -r "$SOURCE_DATE_EPOCH" "$arg" 2>/dev/null ||
+  $my_cmd "$arg"