aumix: fix build against fno-common toolchains
aumix: fix build against fno-common toolchains

Pull Gentoo fix to avoid build failures like:

    $ nix build --impure --expr 'with import ./.{}; aumix.override { stdenv = clang12Stdenv; }' -L
    ...
    aumix> ld: mouse.o:/build/aumix-2.9.1/src/./common.h:132:
      multiple definition of `ourlevel'; common.o:/build/aumix-2.9.1/src/./common.h:132: first defined here

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs