Without the change build fails on -fno-common tollchains as:
$ nix build --impure --expr 'with import ~/nm {}; wmfs.override { stdenv = clang13Stdenv; }'
...
ld: src/log.o:(.bss+0x0): multiple definition of `W'; src/barwin.o:(.bss+0x0): first defined here
ld: src/wmfs.o:(.bss+0x0): multiple definition of `W'; src/barwin.o:(.bss+0x0): first defined here
ld: src/wmfs.o:(.bss+0x10): multiple definition of `event_handle'; src/client.o:(.bss+0x10): first defined here
ff9c57fbed