This package system was designed to work with ipkg-buildpackage (but it has,
admittedly, not actually been tested with that command).
If this is not available (or doesn't work), the process goes like this:

ipkg/rules build
ipkg/rules install
ipkg-build -o root -g root build

optionally, start and/or end the process with ipkg/rules clean

After ipkg-build, you should have gotten a sgt-puzzles_VERSION_armv4t.ipk file,
which is the newly built package.

Building this package requires the dev packages for GTK+ in the toolchain to
be installed. This should be as simple as opkg-target install gtk+-dev
(after running environment-setup). It might not really need all of it, but
installing everything like that is easier than tracking down what it needs.

For the very first build, or if you need to rebuild the icons, you're going to
need cc to be a compiler for the local system, and to have an X server running.
You'll also need the GTK+ dev packages for your local system to be installed.
Don't be surprised if during this process you see a window pop up and then
disappear for each of the games, this is normal when building the icons.