Module libraries: Add dynamic lookup flag for Darwin modules

Modules are used for dynamic plugins, which may have undefined references if they use the APIs of other libraries.

On linux, it is enough to mark these other libraries as OBJECT and the plugin as MODULE and it will just work. On darwin on top of that we need to add a linker specific flag -undefined dynamic_lookup.

I think this should be enabled for MODULES by default to exclude platform specific code from plugins cmake lists.