No stop it! Don't defend autoconf, it is not worth it and the software does not deserve it. Users of autoconf, that is developers that try to use that POS to build their software definitely experiences its pains.
It is not just that M4 is a horribly ugly, badly, inconsistent language. You also have about a million leaky caches in the build process to deal with. Autoconf rewrites the build scripts in three stages, each which is cached and also keeps an autom4te.cache directory around. So what happens is that you update something in the M4 build script and then have to spend hours hunting down why the change didn't change anything because some stupid autoconf cache wasn't flushed.
What autoconf has done is to separate free software developers into two categories. Those who can hack the source (the .c and .h files) and those who understand the "autoconf magic." It should be telling that people refer to it as "magic" - it's a fing build system, not rocket science.
The only reason this ugly piece of software has lived on is because people treat it as some kind of magical device and inertia. Configuring and building software is an easy problem. It gets even easier if you finally decide to drop support for 30 year old unices that no one uses. The autoconf team consistently refuses to do that which why the software wont ever become better.
CMake, SCons, Waf and probably half a dozen more tools all do a much better job configuring software than autoconf ever will. People really need to try those alternatives and I'm sure they will realize autoconf needs to die, die, DIE.