Platform checks with config.h

Hi there,

I was working on getting config.h working with CMake. From looking at
the autoconf-generated config.h.in, I noticed a quite a few #defines
that didn't seem to appear in the source code. In fact, I can't seem to
tell where variables such as HAVE_LIBREADLINE appear in the source.

Am I missing something, or has that not been implemented? Which defines
are actually needed in config.h?

Thanks,
Kevin Zheng