test defs: hack to support autoconf-wrapper programs

test defs: hack to support autoconf-wrapper programs

* tests/defs.in ($AUTOCONF): Add a dummy `-B' option to the
autoconf invocation, so that, when the Debian autoconf wrapper
is involved, it will correctly dispatch an autoconf >= 2.50
instead of defaulting to autoconf 2.13.
($AUTOHEADER, $AUTORECONF): Likewise, but for autoheader and
autoreconf respectively.

Reported by Bruno Haible:
<http://lists.gnu.org/archive/html/automake/2011-12/msg00039.html>