[CocoaPods/CocoaPods#1249] Separate header paths for different targets in .xcconfig files.

This is an update to an earlier pull request #1590 for issue #904. That issue was merged into #1249, so I have relabelled the pull request.

It is a fix for a bug where the header search paths for dependencies for one platform would leak into targets for another platform. For example, if a project has an iOS target and an OS X target, and the OS X target includes an OS X-only dependency, its OS X-only headers would be cross-linked into the iOS project.