Among other things fixes build against -fno-common toolchains
like clang-12:
$ nix build --impure --expr 'with import ./.{}; artha.override { stdenv = clang12Stdenv; }' -L
...
artha> ld: gui.o:/build/artha-1.0.3/src/./mod_notify.h:37:
multiple definition of `notify_uninit';
mod_notify.o:/build/artha-1.0.3/src/./mod_notify.h:37: first defined here