(Mac) Plugin implementation build support

The build system for the plugin needs some slight modification for Mac OS X. I need to add a macosx directory in the plugin, and the npjp2 build has its own Mac-specific changes.
Expanding the scope of this bug to cover the initial checkin of the code for the Mac port. This touches common code mainly to support new interprocess events and a different method for cross-process drawing than what we have now.
Mac build changes now checked in.