Consistent autoconf version

[David DeHaven]

>>>> current AC_PREREQ is 2.61, fwiw, which is from 2006. Latest Autoconf is 2.69 from 2012, preceded by 2.68 from 2010.
>>>> 
>>>> It may be useful to limit the variety of autoconfs in use by setting AC_PREREQ to, say, 2.68.
>>> Eeek! 2.61! I believe our current minimum is 2.67 (though 2.68 seems common and reasonable)
>> Yeah - http://hg.openjdk.java.net/jdk8/jdk8/file/056398db9dcb/common/autoconf/configure.ac
>> says        33 AC_PREREQ([2.61])
>> so bumping it to 2.68 (or at least 2.67) may be reasonable.
>> 
>> cheers,
>> dalibor topic
>> 
> I just checked around on the various systems I'm usually building on. 2.67 is default on my very old Ubuntu. The new Ubuntu I'm switching to has 2.69. On cygwin you get 2.69, on Solaris I used OpenCSW which also provided 2.69. On mac I have only managed to get autoconf through building a source bundle anyway.

MacPorts is at 2.69.

-DrD-