Ah cool, thanks again.
Here is the output, hopefully it points to the problem:
So it seems there is a problem in the model selector. I did actually fix a crash in the model selector quite recently, so building the version from my repository might be worth trying since it will have this fix.
Yes I installed the old package from August 2017 because I could not compile DarkRadiant. The build can not completed. I already ceated a bugreport for it.
The build commands in that bug report do not relate to building DarkRadiant. It seems like you are building pybind11 but inside the DarkRadiant directory, I'm guessing because those build steps were listed in the DarkRadiant Compilation Guide on the Wiki. I have no idea why those commands are suggested or why it would be necessary to build pybind11 before building DarkRadiant; it is certainly not something I have ever done. Perhaps building pybind11 manually on Arch is needed to get Python support working, but Python is optional in DarkRadiant, so I would just skip it for now unless you specifically want to test Python scripts.
To build DarkRadiant, you should only need the commands listed in the README under the heading Compiling on Linux.
sudo make install
or if you just want to make a test build and not install it system-wide, you can pass a different prefix directory e.g.: