perl-cross: fix . being included in INC
perl-cross: fix . being included in INC

perl-cross set `default_inc_excludes_dot` to undefined by default. This
sets `-Ddefault_inc_excludes_dot` explicitly when cross compiling.

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