dbus: fix launchd support on Darwin

- Meson tries to detect that `launchctl` is present. Lie to it because
  it will always be present on a Darwin host platform; and
- Install the launch agent plist to `$out` instead of an impure path.

pkgs/by-name/db/dbus/package.nix

@@ -98,6 +98,7 @@ stdenv.mkDerivation (finalAttrs: {
     "-Dinstall_sysconfdir=${placeholder "out"}/etc"
     "-Ddoxygen_docs=disabled"
     "-Dducktype_docs=disabled"
+    "-Dlaunchd_agent_dir=${placeholder "out"}/Library/LaunchAgents"
     "-Dqt_help=disabled"
     "-Drelocation=disabled" # Conflicts with multiple outputs
     "-Dmodular_tests=disabled" # Requires glib
@@ -114,6 +115,13 @@ stdenv.mkDerivation (finalAttrs: {
     (lib.mesonEnable "systemd" enableSystemd)
     "-Dselinux=disabled"
   ]
+  ++ lib.optionals stdenv.hostPlatform.isDarwin [
+    # `launchctl` is only needed at runtime. Lie to `find_program` because it will always be present on a Darwin host.
+    "--cross-file=${writeText "darwin.ini" ''
+      [binaries]
+      launchctl = 'true'
+    ''}"
+  ]
   ++ lib.optionals enableSystemd [
     "--cross-file=${writeText "crossfile.ini" ''
       [binaries]