ENH: Merged implementations of ConvertToRelative*Path methods.  The main ConvertToRelativePath method is now in cmGlobalGenerator.  It converts paths only if they are at least inside the deepest common directory between the top-level source and build trees.  Each cmLocalGenerator instance calls this global method with its own output directory as the "local" argument from which paths are relative.  Added separate ConvertToOptionallyRelative path that pays attention to the CMAKE_USE_RELATIVE_PATHS option.