doc: HACKING: update to reflect use of newer versions of the GNU Autotools

[salewski]

Back in issue #79 we updated the versions of the GNU Autotools used to build the ads-github-tools project, but neglected to update the documentation in the HACKING file.

[salewski]

The fix for this issue has landed on the master branch. The change will be part of the ads-github-tools-0.3.5 release, when it is published at some point in the future.