Add on autoconf as a dependency when building on macOS #20678

When trying to build the project locally on the latest version of macOS Big Sur, we need autoconf in order to successfully run ./autogen.sh.

Below is when I first saw the error when trying to build Bitcoin Core locally:

bitcoin % ./autogen.sh 
configuration failed, please install autoconf first

This is a nonfunctional change; only updated the build-osx.md file that way all the dependencies are acquired in the first place.

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--174a7506f384e20aa4161008e828411d-->

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #18077 (net: Add NAT-PMP port forwarding support by hebasto)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Normally, automake would pull in autoconf as a dependency. Did this change recently?

Normally, automake would pull in autoconf as a dependency. Did this change recently?

That is correct, I've just verified on macOS BigSur 11.1 (20C69):

% brew install automake
Updating Homebrew...
==> Downloading https://homebrew.bintray.com/bottles/autoconf-2.69.big_sur.bottle.4.tar.gz
Already downloaded: /Users/hebasto1/Library/Caches/Homebrew/downloads/0e5da6f626a43aad805e44fb6cfecf3151b06c1bfa39bc1b727040f30eb39136--autoconf-2.69.big_sur.bottle.4.tar.gz
==> Downloading https://homebrew.bintray.com/bottles/automake-1.16.3.big_sur.bottle.tar.gz
==> Downloading from https://d29vzk4ow07wi7.cloudfront.net/b19be0f4672d3ed2c258eee5f676d27429e5da189c80dc04ba8d01bc44ead320?response-content-disposition=attachment%3Bfilename%3D%22automake-1.16.3.big_sur.bottle.tar.gz%22&Policy=eyJTdGF0ZW1lbnQiOiBbeyJSZXNvdXJjZSI6Imh0dH
######################################################################## 100.0%
==> Installing dependencies for automake: autoconf
==> Installing automake dependency: autoconf
==> Pouring autoconf-2.69.big_sur.bottle.4.tar.gz
🍺  /usr/local/Cellar/autoconf/2.69: 68 files, 3.0MB
==> Installing automake
==> Pouring automake-1.16.3.big_sur.bottle.tar.gz
🍺  /usr/local/Cellar/automake/1.16.3: 131 files, 3.4MB

(deleting this post, was based on a misunderstanding)

The brew page confirms that the dependency still exists: https://formulae.brew.sh/formula/automake#default

So this must have been a glitch. For example if for some reason your automake install was not successful, and neither it and its dependency autoconf was installed, you'd get exactly the same message.