build: use date.py instead of date

MacOS (and others) "date" command don't support all the options that GNU
date has. So don't depend on that, add a small python program for this.

build-aux/date.py

@@ -0,0 +1,9 @@
+#!/bin/env python
+#The MacOS 'date' is not quite up to GNU standards
+import sys
+from datetime import datetime
+date=datetime.strptime(sys.argv[1],'%Y-%m-%d')
+print(date.strftime(sys.argv[2]))
+#
+#  ./date.py 2023-10-14 "The year-month is %y %m"
+#