[CocoaPods/CocoaPods#904] Separate header paths for different targets in .xcconfig files.

This patch changes how header paths are created in .xcconfig files such that if the podfile has two targets for different platforms (e.g. iOS and OS X), then it won't end up causing the iOS target to reference headers from pods in the OS X target.

We encountered this problem when using Chameleon, because it replaces UIKit headers -- but the iOS target ended up linking against the (OS X only!) Chameleon headers.

This is my first contribution to CocoaPods and I haven't fully verified that it is correct, but it does work for our purposes. Please double-check I haven't broken anything before merging.