allegro5: split outputs -> out, dev

This avoids referencing apple-sdk in the main output on macOS/Darwin,
which saves over a gigabyte of closure size on programs built with
Allegro 5.

Resolves #397472.

pkgs/development/libraries/allegro/5.nix

@@ -100,6 +100,11 @@ stdenv.mkDerivation rec {
 
   cmakeFlags = [ "-DCMAKE_SKIP_RPATH=ON" ];
 
+  outputs = [
+    "out"
+    "dev"
+  ];
+
   meta = with lib; {
     description = "Game programming library";
     homepage = "https://liballeg.org/";