plAudioCore needs to know whether we intend to build the plugin, so
the change introduced in 978edc1 was causing compilation to fail when
building the plugin.
This moves detection and definition back into the root CMakeList,
but preserves the behavior of only looking for the SDK if needed.
So, yeah, this blows away MaxSceneViewer. We're never going to get around
to fixing that. In exchange for this massive code explosion, you get a
free MaxFileData stub type thing. It keeps all the data just so we don't
blow up Cyan's plugins. This fixes the "Missing Dll" messages people keep
whining about.