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_events/messages/juce_Message.h

@@ -0,0 +1,64 @@
+/*
+  ==============================================================================
+
+   This file is part of the JUCE library.
+   Copyright (c) 2017 - ROLI Ltd.
+
+   JUCE is an open source library subject to commercial or open-source
+   licensing.
+
+   The code included in this file is provided under the terms of the ISC license
+   http://www.isc.org/downloads/software-support-policy/isc-license. Permission
+   To use, copy, modify, and/or distribute this software for any purpose with or
+   without fee is hereby granted provided that the above copyright notice and
+   this permission notice appear in all copies.
+
+   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
+   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
+   DISCLAIMED.
+
+  ==============================================================================
+*/
+
+namespace juce
+{
+
+class MessageListener;
+
+
+//==============================================================================
+/** The base class for objects that can be sent to a MessageListener.
+
+    If you want to send a message that carries some kind of custom data, just
+    create a subclass of Message with some appropriate member variables to hold
+    your data.
+
+    Always create a new instance of a Message object on the heap, as it will be
+    deleted automatically after the message has been delivered.
+
+    @see MessageListener, MessageManager, ActionListener, ChangeListener
+
+    @tags{Events}
+*/
+class JUCE_API  Message  : public MessageManager::MessageBase
+{
+public:
+    //==============================================================================
+    /** Creates an uninitialised message. */
+    Message() noexcept;
+    ~Message();
+
+    using Ptr = ReferenceCountedObjectPtr<Message>;
+
+    //==============================================================================
+private:
+    friend class MessageListener;
+    WeakReference<MessageListener> recipient;
+    void messageCallback() override;
+
+    // Avoid the leak-detector because for plugins, the host can unload our DLL with undelivered
+    // messages still in the system event queue. These aren't harmful, but can cause annoying assertions.
+    JUCE_DECLARE_NON_COPYABLE (Message)
+};
+
+} // namespace juce