Clean configure when before invoking autoconf

Autoconf doesn't remake configure if creation time on configure.in
is older than the previous version.

This caused problems on windows so always recreate configure when
running ./otp_build autoconf

otp_build

@@ -310,6 +310,11 @@ do_autoconf ()
 		echo "=== cleaning $d/autom4te.cache"
 		rm -f "$d"/autom4te.cache/*
 	    }
+	    [ ! -f "$d/configure" ] || {
+		echo "=== cleaning $d/configure"
+		rm -f "$d"/configure
+	    }
+
 	    echo "=== running autoconf in $d"
 	    ( cd "$d" && autoconf ) || exit 1
 	    chdr=`cat "$file" | sed -n "s|.*\(AC_CONFIG_HEADER\).*|\1|p"`