Changes to install in Mac

I don't actually expect this branch to be merged, but it is only to explain what I'm currently doing to be able to generate a Mac installer:
1. Change executable extension according to platform
2. Changing OSG_LIBRARY_PATH to `/vendor/lib/osgPlugins-3.0.1`

I actually don't put the `osgconv` and `osgviewer` executables in `/osgconv`, but instead simlinks to the real files that are in `vendor/bin`.