fix macOS build (following Projucer changes made in Windows, which removed /Applications/JUCE/modules from its headers). move JUCE headers under source control, so that Windows and macOS can both build against same version of JUCE. remove AUv3 target (I think it's an iOS thing, so it will never work with this macOS fluidsynth dylib).

modules/juce_core/native/java/JuceFirebaseMessagingService.java

@@ -0,0 +1,35 @@
+package com.juce;
+
+import com.google.firebase.messaging.*;
+
+public final class JuceFirebaseMessagingService extends FirebaseMessagingService
+{
+    private native void firebaseRemoteMessageReceived (RemoteMessage message);
+    private native void firebaseRemoteMessagesDeleted();
+    private native void firebaseRemoteMessageSent (String messageId);
+    private native void firebaseRemoteMessageSendError (String messageId, String error);
+
+    @Override
+    public void onMessageReceived (RemoteMessage message)
+    {
+        firebaseRemoteMessageReceived (message);
+    }
+
+    @Override
+    public void onDeletedMessages()
+    {
+        firebaseRemoteMessagesDeleted();
+    }
+
+    @Override
+    public void onMessageSent (String messageId)
+    {
+        firebaseRemoteMessageSent (messageId);
+    }
+
+    @Override
+    public void onSendError (String messageId, Exception e)
+    {
+        firebaseRemoteMessageSendError (messageId, e.toString());
+    }
+}