autoreconf.in: improve compatibility with old perl

autoreconf.in: improve compatibility with old perl

In very old perls (I noticed this with 5.8.4), File::Temp objects are
not automatically stringified in all contexts where we expect them to
be, causing autoreconf to crash.

* bin/autoreconf.in (install_aux_file): Explicitly extract the
  temporary file’s name from $temp, and use that in all the places we
  were using $temp.