Motivation for this change

Builds currently fail with ar trying to operate on what are clearly two paths concatenated together. It stems from a backward-incompatible change in Make:

Previously appending using '+=' to an empty variable would result in a value starting with a space. Now the initial space is only added if the variable already contains some value. Similarly, appending an empty string does not add a trailing space.

This issue was first reported on mamedev/mame#6248, and affects libretro's 2016 snapshot as well. A fix that is reported to work with previous versions of Make was upstreamed to GENie, the build system, MAME, and now to libretro-mame2016 (pending), from which the patch comes.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

If this gets backported to 20.09, libretro can then be unmarked as broken, as far as I can tell.