I am not a Mac person! The procedure below was worked out with help from a friend who develops for Mac. This did work for me on a Mac running macOS 12.4 (Monterey). Note that a full Xcode install was not needed.
Install Xcode Command Line Tools
You can read about this
here.
I typed git in a terminal and it triggered the install, which took around 20 minutes.
Install XQuartz
From https://www.xquartz.org. Install the package.
Install Brew
$ /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
After the install it will suggest adding some stuff to your shell profile, which I did. See example. Not sure if this was critical.
Install jpeg libraries required by XForms
$ brew install jpeg
$ brew install jpeg-turbo
Install XForms toolkit
From http://xforms-toolkit.org download http://download.savannah.nongnu.org/releases/xforms/xforms-1.2.4.tar.gz.
$ cd xforms-1.2.4
$ ./configure
$ make
$ make install
(/usr/local/lib should now have libjpeg.dylib and the libforms.dylib)
Build the Calculator
$ cd calc-demo-xwin/src
$ make
When you run it with ./calc it takes a while to fire up the X server but then it should run.