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
...
Similar to some other builds, sdl-config gets stuck and times out.
checking for sdl-config... /nix/store/q2hm1ka0qxs2gv5bmrxj7j80lygk4b5z-SDL-1.2.15-dev/bin/sdl-config
building of '/nix/store/m2d7v0n5hd2498vfxp18i37p7r1lf76p-freedroidrpg-0.16.1' timed out after 7200 seconds of silence
/cc ZHF #45961