native_target add_system_library

Hello, I have a question.
In the `native_target.rb`
```
def add_system_library(names)
Array(names).each do |name|
path = "usr/lib/lib#{name}.dylib"
files = project.frameworks_group.files
unless reference = files.find { |ref| ref.path == path }
reference = project.frameworks_group.new_file(path, :sdk_root)
end
frameworks_build_phase.add_file_reference(reference, true)
reference
end
end
```

But when wo use xcode7+, the library name is not *.dylib, but *.tbd
and the path is `/Applications/Xcode.app/Contents/Developer/Platforms/iphoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/lib/*.tbd`
The old implement maybe has a little problem. Can we use it as before？

Start in native_target.rb at add_system_library and inspect how system library paths are constructed and added to the frameworks group. Compare the existing .dylib behavior with the reported Xcode 7+ .tbd paths; done means system libraries are correctly handled for the reported SDK layout.

Written by the indexing model from the issue text.