History of Autoconf
gnu.org
gnu.org
Once you get it running it generally does a solid job. It helps to run your code even on systems that you have never heard of.
On the other hand if you wanted to change your configuration, you sometimes ended up trying to figure out some M4 macro's that had no real documentation.
In a historical context, it was a very smart piece of technology when it was developed, but I am glad that I do not have to use it currently.