GNU autoconf-2.70 released

I guess I understand the impetus behind fine-grained feature detection and verification, but I think it is just a maintenance nightmare[1]. Any new C or C++ compiler these days is going to have to emulate an existing compiler for at least some part of its lifecycle just to get past all the `#error "unknown compiler"` shenanigans that exist out there (once they're big enough, teaching the main build systems out there about your new compiler should have been on the task list by then). It basically means one needs a CMake roughly newer than your compiler, but I don't think that's *too* much of an ask at least.

It would likely mean revving up the autoconf release cycle though since platforms change all the time…

[1]In CMake, we had fine-grained feature detection for C++11, but with 14 and newer, compilers are following the standard pretty closely now and we just have release-level feature requests. These are now just gated by version checks in the compiler detection routines declaring what language versions are supported.