Without the upstream change build fails on toolchains that default to
`-fno-common` (like clang-13 or upstream gcc-10+):
$ nix build --impure --expr 'with import ./.{}; freedroidrpg.override { stdenv = clang13Stdenv; }' -L
...
freedroidrpg> ld: addon_crafting_ui.o:/build/freedroidrpg-0.16.1/src/./struct.h:1025:
multiple definition of `tux_rendering'; action.o:/build/freedroidrpg-0.16.1/src/./struct.h:1025: first defined here
...