If you installed PCManFM on Void Linux or another minimal setup, here are the steps you need to follow to get USB-automounting to work properly:

1. Install the optional dependency gvfs, which should come with udisks2 and a polkit
2. Launch your WM session with exec dbus-run-session WMHERE so all these programs can talk to each other.
3. Make sure the policykit service is running.
4. Add the following file to /etc/polkit-1/rules.d/ as 50-udisks.rules or something:
   polkit.addRule(function(action, subject) {
   if (subject.isInGroup("wheel")) {
        if (action.id.startsWith("org.freedesktop.udisks2.")) {
            return polkit.Result.YES;
        }
    }
});

   Credit to fractalf on GitHub for these concise rules.

5. To enable auto-mounting of connected Android devices, install the gvfs-mtp and android-udev-rules packages.