Autoconf version number after 2.70

[Paul Eggert]

Given the changes being discussed (which seem good ones), I suggest calling 
the next Autoconf release 2.71 not 2.70.1, as the latter would use a 
new-to-Autoconf numbering convention that might be more trouble than it's worth.

There was little difference (and only a month) between Autoconf 2.66 and 
2.67, so there's precedent for putting only a few changes into Autoconf 2.71 
and publishing it relatively quickly.