Earlier quoted context omitted.
./configure is a generated file. It's not committed to the repository. If you download a release (like "guile-3.0.0.tar.gz"), it has the files you're looking for. Or you can learn autotools and do it yourself, but I don't recommend opening that can of worms.
Hmmh, it's been a decade or two since I used the autotools last, so I'll need a hint. I find it slightly disturbing that the README file is of little help though. Further, if I try naively $ sh -x ./autogen.sh then that fails on a dependency on libtoolize, of which I never have heard of and which doesn't seem to be known to bananian.
Agreed that the README needs much better documentation of the package-time dependencies (autoconf, automake, m4, perl, etc), and the build-time dependencies (libtool, libgc, plus a bunch of other libraries).