* build-aux/announce-gen: Add two new options, both optional:

* build-aux/announce-gen: Add two new options, both optional:
--bootstrap-tools=TOOL_LIST
      a comma-separated list of tools, e.g.,
      autoconf,automake,bison,gnulib
--gnulib-snapshot-date=DATE
      if gnulib is in the bootstrap tool list,
      then report this as the snapshot date.
      If not specified, use the current date/time.
      If you specify a date here, be sure it's UTC.