Add support for conditional compile options, add pulseaudio and ffmpeg4
support patches.

Motivation for this change

Unfortunately, previous version of this package was very basic and did not allow for conditional compile options.

Also, it lacked pulseaudio support (it was added with a patch, courtesy of marienz) and it wouldn't work properly with ffmpeg 4 (support also added via patch, see here). I had to change patch formats a bit so that it would work OOTB with nix.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nix-review --run "nix-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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.