getdate.sh: make the date(1) usage more portable

There are several different flavors of date(1) out there.  Try a few
different CLI options for date(1) to see which one works.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

Author: jsquyres

config/getdate.sh

@@ -7,4 +7,8 @@
 # Provide a way to override build date for reproducible build results
 # See https://reproducible-builds.org/ for why this is good.
 
-date --date="@${SOURCE_DATE_EPOCH:-$(date +%s)}" "$@"
+# There are several different flavors of date(1) out there.
+# Try a few different CLI options for date(1) to see which one works.
+
+SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$(date +%s)}"
+date -u -d "@$SOURCE_DATE_EPOCH" "$@" 2>/dev/null || date -u -r "$SOURCE_DATE_EPOCH" "$@" 2>/dev/null || date -u "$@"