Motivation for this change

fluidsynth is compiled in but soundfont-fluid needs to be explicitly
pulled in and path configured, an option makes it much simpler to use

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)

I tried running nixos/tests/mpd.nix but it looks like my hosts' config firewall bled into it? or it doesn't work anymore because of firewall; port 6600 was refused...

serverPulseAudio # [   26.809227] refused connection: IN=eth1 OUT= MAC=52:54:00:12:01:03:52:54:00:12:01
:01:08:00 SRC=192.168.1.1 DST=192.168.1.3 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=27402 DF PROTO=TCP SPT=34
152 DPT=6600 WINDOW=64240 RES=0x00 SYN URGP=0

Anyway I tested manually via build-vm and it works (both pulling the package and config looked ok with & without new option set)

$ cat vm.nix
{ lib, config, ... }:
{
  services.mpd = {
    enable = true;
    fluidsynth = true;
  };
  users.users.root.initialPassword = "root";
}
$ nixos-rebuild -I nixpkgs=/home/asmadeus/nixpkgs/ -I nixos-config=./vm.nix build-vm
...
$ /nix/store/9ljp3igryy56lzk5xf5lvhf24c01ni8p-nixos-vm/bin/run-nixos-vm -serial mon:stdio -nographic
... service running, config ok and file accessible despite
... not having installed it previously on the host

• 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.

mpd maintainers @tobim @fpletz @ehmry @astsmtl