Provide a build setting to disable duplicate library warnings from the linker

[cmcgee1024]

Current, SwiftPM sets the "OTHER_LDFLAGS" build setting to $(inherited) -Wl,-no_warn_duplicate_libraries so that the Apple platform linker won't emit this warning. However, this flag is unknown to linkers other than ld64 for Apple platforms.

There should be a higher-level build setting so that SwiftPM can set it and it takes effect on Apple platforms, and equivalent setting can be used for other linkers.

[jakepetroules]

This was also a workaround for a bug, so ideally we'd fix the underlying issue rather than propagating this further.