This confused me, so I'm documenting my workaround, which is probably not the correct workaround, but it seemed to work (leaving non-persistent state, lol).
When in a shell with mix and clang provided via nix, the file_system package fails to build the mac_listener binary via clang, with the error (reproduced in a shell):
$ clang -framework CoreFoundation -framework CoreServices -Wno-deprecated-declarations c_src/mac/*.c -o priv/mac_listener
In file included from c_src/mac/cli.c:2:
In file included from c_src/mac/cli.h:4: