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_audio_devices/audio_io/juce_SystemAudioVolume.h

@@ -0,0 +1,59 @@
+/*
+  ==============================================================================
+
+   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
+{
+
+//==============================================================================
+/**
+    Contains functions to control the system's master volume.
+
+    @tags{Audio}
+*/
+class JUCE_API  SystemAudioVolume
+{
+public:
+    //==============================================================================
+    /** Returns the operating system's current volume level in the range 0 to 1.0 */
+    static float JUCE_CALLTYPE getGain();
+
+    /** Attempts to set the operating system's current volume level.
+        @param newGain  the level, between 0 and 1.0
+        @returns true if the operation succeeds
+    */
+    static bool JUCE_CALLTYPE setGain (float newGain);
+
+    /** Returns true if the system's audio output is currently muted. */
+    static bool JUCE_CALLTYPE isMuted();
+
+    /** Attempts to mute the operating system's audio output.
+        @param shouldBeMuted    true if you want it to be muted
+        @returns true if the operation succeeds
+    */
+    static bool JUCE_CALLTYPE setMuted (bool shouldBeMuted);
+
+private:
+    SystemAudioVolume(); // Don't instantiate this class, just call its static fns.
+    JUCE_DECLARE_NON_COPYABLE (SystemAudioVolume)
+};
+
+} // namespace juce