From the README:

This is v4.1.0 of calctool, the calculator application that was in the
OpenWindows Deskset of the Solaris 8 operating system. Sun Microsystems Inc.
have kindly given me permission to release it.

It incorporates a multiple precision arithmetic packages based on the work
of Professor Richard Brent, who has also kindly given me permission to make
it available.

There is a single graphics driver for Gtk2 included with this release, but
the code is written in a portable abstracted way, so it should not be very
difficult to add others over time.