Rejuvenating Autoconf

I heard a story about Go developers inspecting C APIs by doing `clang -Dkeyword=garbage`, compiling the source and parsing error messages to determine what the API surface was actually like. They then had the gall to be offended when the stdout format was changed and their scraper no longer worked. So they still care about interface stability, but their allergies to non-Go code makes the latch onto the weirdest appendages instead. It's not like things such as castxml don't exist…