The person (Arun Persaud) who used to update the "official" xboard source on
savannah.gnu.org by merging in HGM's work mostly left the project several years ago, and I haven't been active in the project for many years other than to do occasional administrative things when needed. So I assume HGM's source is quite a ways ahead of what's on Savannah. Looking at the git log, I see the source on Savannah has not changed much since v4.9.1. was tagged.
I actually don't know how to git clone HGM's source. I'm not sure if he has a git server or only gitweb. I wasn't able to guess the right URL to clone from via git:// or http://. HGM, if you're tuned in, can you answer that?
I see the compile instructions in the README are out of date for starting from a git clone. They are missing the initial step of running ./autogen.sh. I tried the following on my current Ubuntu 22.04 system, which I think is more or less correct.
./autogen.sh
./configure
make
However, the current git source at the head of the master branch ultimately fails to link with lots of "multiple definition" errors on ics_type and currPvInfo. Maybe an incompatibility with the current gcc.
You may not even get that far if you don't have enough packages installed on your Linux. On Ubuntu, "apt get build-essential" helps, but there are more developer packages needed that build-essential doesn't pull in.