Issue with pod lib lint when podspec uses prefix_header_*

Issue with pod lib lint when podspec uses prefix_header_*

Hey,

Just ran into this problem with pod lib lint that fails to properly lint a pod when it defines a prefix_header content/file.
The source of the problem is actually quite simple: lib lint works from /tmp which is actually a symlink to /private/tmp (on at least Mountain Lion and Mavericks, probably true before too). The sandbox is created with a Pathname of "/tmp/Cocoapods/Lint", and there are a few usages of relative_path_from, which assumes that no symlinks exist between the target and the receiver.
That causes the prefix file reference to be short one "../" and Xcode barfs during the final libPod build since it can't reach said prefix file.

I've also created a sample project to help demonstrate/test the problem: https://github.com/opentable/PodLibBug

/kra