Fails to correct paths from "hidden" Xcode projects

[RJVB]

Xcode projects do not have to be in the project root directory, but can be in a subdirectory.
For example, VLC ships a vlc.xcodeproj as extras/package/macosx/vlc.xcodeproj.
mulle-xcode-to-cmake creates a CMakeSourcesAndHeaders.txt file that contains paths that are
relative to the .xcodeproj file's directory, instead of relative to the toplevel directory
(if that's where mulle-xcode-to-cmake is invoked).