make it build on M2 macOS 13 (probably M1 and 10.13)

[atsushieno]

I had been trying to build ADLplug/OPNplug on my M2 mac and managed to get it working, so I would share it here.

There was a drawback in the change set though; midi_keyboard_ex.h/.cc does not compile with the latest clang++ from Xcode toolchains (it fails to resolve the class name at JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR). Looks like it is required only to reduce juce_audio_utils, so I only removed the hack around (ex namespace) and simply added the module to the build as I don't see the reason why it had better be cut. If there is any way to preserve it to still compile fine, then I would bring it back, I do not stick to it.